diff --git a/luametalatex-back-pdf.lua b/luametalatex-back-pdf.lua
index d80264e..74afa30 100644
--- a/luametalatex-back-pdf.lua
+++ b/luametalatex-back-pdf.lua
@@ -8,6 +8,7 @@ local fontdirs = setmetatable({}, {__index=function(t, k)t[k] = pfile:getobj() r
 local usedglyphs = {}
 local dests = {}
 local cur_page
+local declare_whatsit = require'luametalatex-whatsits'.new
 local whatsit_id = node.id'whatsit'
 local whatsits = node.whatsits()
 local colorstacks = {{
@@ -264,9 +265,16 @@ local function linkcontext_set(linkcontext, p, x, y, list, level, kind)
   end end
 end
 
-function do_start_link(prop, p, n, x, y, outer, _, level)
+local start_link_whatsit = declare_whatsit('pdf_start_link', function(prop, p, n, x, y, outer, _, level)
+  if not prop then
+    tex.error('Invalid pdf_start_link whatsit', {"A pdf_start_link whatsit did not contain all necessary \z
+        parameters. Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+    return
+  end
   if not p.is_page then
-    error[[No link allowed here]]
+    tex.error('pdf_start_link outside of page', {"PDF links are not allowed in Type3 charstrings or Form XObjects. \z
+        The link will be ignored"})
+    return
   end
   local links = p.linkcontext
   if not links then
@@ -276,29 +284,45 @@ function do_start_link(prop, p, n, x, y, outer, _, level)
   local link = {quads = {}, attr = prop.link_attr, action = prop.action, level = level, force_separate = false} -- force_separate should become an option
   links[#links+1] = link
   addlinkpoint(p, link, x, y, outer, 'start')
-end
-function do_end_link(prop, p, n, x, y, outer, _, level)
+end)
+local end_link_whatsit = declare_whatsit('pdf_end_link', function(prop, p, n, x, y, outer, _, level)
   if not p.is_page then
-    error[[No link allowed here]]
+    tex.error('pdf_start_link outside of page', {"PDF links are not allowed in Type3 charstrings or Form XObjects. \z
+        The link will be ignored"})
+    return
   end
   local links = p.linkcontext
-  if not links then error"No link here to end" end
+  if not links then
+    tex.error('No link here to end', {"You asked me to end a link, but currently there is no link active. \z
+        Maybe you forgot to run \\pdfextension startlink first?"})
+    return
+  end
   local link = links[#links]
+  if link.level ~= level then
+    tex.error('Inconsistent link level', {"You asked me to end a link, but the most recent link had been started at another level. \z
+        I will continue with the link for now."})
+    return
+  end
   links[#links] = nil
   if not links[1] then p.linkcontext = nil end
-  if link.level ~= level then error"Wrong link level" end
   addlinkpoint(p, link, x, y, outer, 'final')
-end
+end)
 
-local do_setmatrix do
+local setmatrix_whatsit do
   local numberpattern = (lpeg.P'-'^-1 * lpeg.R'09'^0 * ('.' * lpeg.R'09'^0)^-1)/tonumber
   local matrixpattern = numberpattern * ' ' * numberpattern * ' ' * numberpattern * ' ' * numberpattern
-  function do_setmatrix(prop, p, n, x, y, outer)
+  setmatrix_whatsit = declare_whatsit('pdf_setmatrix', function(prop, p, n, x, y, outer)
+    if not prop then
+      tex.error('Invalid pdf_setmatrix whatsit', {"A pdf_setmatrix whatsit did not contain a matrix value. \z
+          Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+      return
+    end
     local m = p.matrix
     local a, b, c, d = matrixpattern:match(prop.data)
     if not a then
-      print(prop.data)
-      error[[No valid matrix found]]
+      tex.error('Invalid matrix', {"The matrix in this pdf_setmatrix whatsit does not have the expected structure and could not be parsed. \z
+          Did you provide enough parameters? The matrix needs exactly four decimal entries."})
+      return
     end
     local e, f = (1-a)*x-c*y, (1-d)*y-b*x -- Emulate that the origin is at x, y for this transformation
                                           -- (We could also first translate by (-x, -y), then apply the matrix
@@ -308,19 +332,23 @@ local do_setmatrix do
     c, d = projected(m, c, d, 0)
     e, f = projected(m, e, f, 1)
     m[1], m[2], m[3], m[4], m[5], m[6] = a, b, c, d, e, f
-  end
+  end)
 end
-local function do_save(prop, p, n, x, y, outer)
+local save_whatsit = declare_whatsit('pdf_save', function(prop, p, n, x, y, outer)
   pdf.write('page', 'q', x, y, p)
   local lastmatrix = p.matrix
   p.matrix = {[0] = lastmatrix, table.unpack(lastmatrix)}
-end
-local function do_restore(prop, p, n, x, y, outer)
+end)
+local restore_whatsit = declare_whatsit('pdf_restore', function(prop, p, n, x, y, outer)
   -- TODO: Check x, y
   pdf.write('page', 'Q', x, y, p)
   p.matrix = p.matrix[0]
-end
-local function do_dest(prop, p, n, x, y)
+end)
+local dest_whatsit = declare_whatsit('pdf_dest', function(prop, p, n, x, y)
+  if not prop then
+    tex.error('Invalid pdf_dest whatsit', {"A pdf_dest whatsit did not contain all necessary \z
+        parameters. Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+  end
   assert(cur_page, "Destinations can not appear outside of a page")
   local id = prop.dest_id
   local dest_type = prop.dest_type
@@ -365,14 +393,29 @@ local function do_dest(prop, p, n, x, y)
   else
     dests[id] = pfile:indirect(dests[id], data)
   end
-end
-local function do_refobj(prop, p, n, x, y)
+end)
+local refobj_whatsit = declare_whatsit('pdf_refobj', function(prop, p, n, x, y)
+  if not prop then
+    tex.error('Invalid pdf_refobj whatsit', {"A pdf_refobj whatsit did not reference any object. \z
+        Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+    return
+  end
   pfile:reference(prop.obj)
-end
-local function do_literal(prop, p, n, x, y)
+end)
+local literal_whatsit = declare_whatsit('pdf_literal', function(prop, p, n, x, y)
+  if not prop then
+    tex.error('Invalid pdf_literal whatsit', {"A pdf_literal whatsit did not contain a literal to be inserted. \z
+        Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+    return
+  end
   pdf.write(prop.mode, prop.data, x, y, p)
-end
-local function do_colorstack(prop, p, n, x, y)
+end)
+local colorstack_whatsit = declare_whatsit('pdf_colorstack', function(prop, p, n, x, y)
+  if not prop then
+    tex.error('Invalid pdf_colorstack whatsit', {"A pdf_colorstack whatsit did not contain all necessary \z
+        parameters. Maybe your code hasn't been adapted to LuaMetaLaTeX yet?"})
+    return
+  end
   local colorstack = prop.colorstack
   local stack
   if p.is_page then
@@ -393,28 +436,33 @@ local function do_colorstack(prop, p, n, x, y)
     stack[#stack] = prop.data
   end
   pdf.write(colorstack.mode, stack[#stack], x, y, p)
-end
+end)
 local function write_colorstack()
   local idx = token.scan_int()
   local colorstack = colorstacks[idx + 1]
   if not colorstack then
-    error[[Undefined colorstack]]
+    tex.error('Undefined colorstack', {"The requested colorstack is not initialized. \z
+        This probably means that you forgot to run \\pdffeedback colorstackinit or \z
+        that you specified the wrong index. I will continue with colorstack 0."})
+    colorstack = colorstacks[1]
   end
   local action = token.scan_keyword'pop' and 'pop'
               or token.scan_keyword'set' and 'set'
               or token.scan_keyword'current' and 'current'
               or token.scan_keyword'push' and 'push'
   if not action then
-    error[[Missing action specifier for colorstack command]]
+    tex.error('Missing action specifier for colorstack', {
+        "I don't know what you want to do with this colorstack. I would have expected pop/set/current or push here. \z
+        I will ignore this command."})
+    return
   end
   local text
   if action == "push" or "set" then
     text = token.scan_string()
     -- text = token.to_string(token.scan_tokenlist()) -- Attention! This should never be executed in an expand-only context
   end
-  local whatsit = node.new(whatsit_id, whatsits.pdf_colorstack)
+  local whatsit = node.new(whatsit_id, colorstack_whatsit)
   node.setproperty(whatsit, {
-      handle = do_colorstack,
       colorstack = colorstack,
       action = action,
       data = text,
@@ -498,52 +546,40 @@ token.luacmd("pdfextension", function(_, imm)
   elseif token.scan_keyword"literal" then
     local mode = scan_literal_mode()
     local literal = token.scan_string()
-    local whatsit = node.new(whatsit_id, whatsits.pdf_literal)
+    local whatsit = node.new(whatsit_id, literal_whatsit)
     node.setproperty(whatsit, {
-        handle = do_literal,
         mode = mode,
         data = literal,
       })
     node.write(whatsit)
   elseif token.scan_keyword"startlink" then
     local pfile = get_pfile()
-    local whatsit = node.new(whatsit_id, whatsits.pdf_start_link)
+    local whatsit = node.new(whatsit_id, start_link_whatsit)
     local attr = token.scan_keyword'attr' and token.scan_string() or ''
     local action = scan_action()
     local objnum = pfile:getobj()
     lastannot = num
     node.setproperty(whatsit, {
-        handle = do_start_link,
         link_attr = attr,
         action = action,
         objnum = objnum,
       })
     node.write(whatsit)
   elseif token.scan_keyword"endlink" then
-    local whatsit = node.new(whatsit_id, whatsits.pdf_end_link)
-    node.setproperty(whatsit, {
-        handle = do_end_link,
-      })
+    local whatsit = node.new(whatsit_id, end_link_whatsit)
     node.write(whatsit)
   elseif token.scan_keyword"save" then
-    local whatsit = node.new(whatsit_id, whatsits.pdf_save)
-    node.setproperty(whatsit, {
-        handle = do_save,
-      })
+    local whatsit = node.new(whatsit_id, save_whatsit)
     node.write(whatsit)
   elseif token.scan_keyword"setmatrix" then
     local matrix = token.scan_string()
-    local whatsit = node.new(whatsit_id, whatsits.pdf_setmatrix)
+    local whatsit = node.new(whatsit_id, setmatrix_whatsit)
     node.setproperty(whatsit, {
-        handle = do_setmatrix,
         data = matrix,
       })
     node.write(whatsit)
   elseif token.scan_keyword"restore" then
-    local whatsit = node.new(whatsit_id, whatsits.pdf_restore)
-    node.setproperty(whatsit, {
-        handle = do_restore,
-      })
+    local whatsit = node.new(whatsit_id, restore_whatsit)
     node.write(whatsit)
   elseif token.scan_keyword"info" then
     infodir = infodir .. token.scan_string()
@@ -577,10 +613,9 @@ token.luacmd("pdfextension", function(_, imm)
     end
   elseif token.scan_keyword"refobj" then
     local num = token.scan_int()
-    local whatsit = node.new(whatsit_id, whatsits.pdf_refobj)
+    local whatsit = node.new(whatsit_id, refobj_whatsit)
     node.setproperty(whatsit, {
         obj = num,
-        handle = do_refobj,
       })
     node.write(whatsit)
   elseif token.scan_keyword"outline" then
@@ -616,10 +651,9 @@ token.luacmd("pdfextension", function(_, imm)
     else
       error[[Unsupported id type]]
     end
-    local whatsit = node.new(whatsit_id, whatsits.pdf_dest)
+    local whatsit = node.new(whatsit_id, dest_whatsit)
     local prop = {
       dest_id = id,
-      handle = do_dest,
     }
     node.setproperty(whatsit, prop)
     if token.scan_keyword'xyz' then
diff --git a/luametalatex-firstcode.lua b/luametalatex-firstcode.lua
index 33a97d5..15318b8 100644
--- a/luametalatex-firstcode.lua
+++ b/luametalatex-firstcode.lua
@@ -63,50 +63,8 @@ end
 --     end
 --   })
 
+local new_whatsit = require'luametalatex-whatsits'.new
 local whatsit_id = node.id'whatsit'
-local whatsits = {
-  [0] = "open",
-        "write",
-        "close",
-        "special",
-        nil,
-        nil,
-        "save_pos",
-        "late_lua",
-        "user_defined",
-        nil,
-        nil,
-        nil,
-        nil,
-        nil,
-        nil,
-        nil,
-        "pdf_literal",
-        "pdf_refobj",
-        "pdf_annot",
-        "pdf_start_link",
-        "pdf_end_link",
-        "pdf_dest",
-        "pdf_action",
-        "pdf_thread",
-        "pdf_start_thread",
-        "pdf_end_thread",
-        "pdf_thread_data",
-        "pdf_link_data",
-        "pdf_colorstack",
-        "pdf_setmatrix",
-        "pdf_save",
-        "pdf_restore",
-}
-whatsits[whatsits[0]] = 0
-for i = 0,#whatsits do
-  local v = whatsits[i]
-  if v then
-    whatsits[v] = i
-  end
-end
-function node.whatsits() return whatsits end
-function node.subtype(s) return type(s) == "string" and whatsits[s] or nil end
 local spacer_cmd, relax_cmd = token.command_id'spacer', token.command_id'relax'
 local function scan_filename()
   local name = {}
@@ -141,15 +99,16 @@ local function do_openout(p)
     end
   end
 end
+local open_whatsit = new_whatsit('open', do_openout)
 token.luacmd("openout", function(_, immediate) -- \openout
   local file = token.scan_int()
   token.scan_keyword'='
   local name = scan_filename()
-  local props = {file = file, name = name, handle = do_openout}
+  local props = {file = file, name = name}
   if immediate == "immediate" then
     do_openout(props)
   else
-    local whatsit = node.direct.new(whatsit_id, whatsits.open)
+    local whatsit = node.direct.new(whatsit_id, open_whatsit)
     properties[whatsit] = props
     node.direct.write(whatsit)
   end
@@ -160,13 +119,14 @@ local function do_closeout(p)
     ofiles[p.file] = nil
   end
 end
+local close_whatsit = new_whatsit('close', do_closeout)
 token.luacmd("closeout", function(_, immediate) -- \closeout
   local file = token.scan_int()
-  local props = {file = file, handle = do_closeout}
+  local props = {file = file}
   if immediate == "immediate" then
     do_closeout(props)
   else
-    local whatsit = node.direct.new(whatsit_id, whatsits.close)
+    local whatsit = node.direct.new(whatsit_id, close_whatsit)
     properties[whatsit] = props
     node.direct.write(whatsit)
   end
@@ -180,14 +140,15 @@ local function do_write(p)
     texio.write_nl(p.file < 0 and "log" or "term and log", content)
   end
 end
+local write_whatsit = new_whatsit('write', do_write)
 token.luacmd("write", function(_, immediate) -- \write
   local file = token.scan_int()
   local content = token.scan_tokenlist()
-  local props = {file = file, data = content, handle = do_write}
+  local props = {file = file, data = content}
   if immediate == "immediate" then
     do_write(props)
   else
-    local whatsit = node.direct.new(whatsit_id, whatsits.write)
+    local whatsit = node.direct.new(whatsit_id, write_whatsit)
     properties[whatsit] = props
     node.direct.write(whatsit)
   end
@@ -199,7 +160,7 @@ token.luacmd("immediate", function() -- \immediate
     return token.put_next(next_tok)
   end
   local function_id = next_tok.mode
-  functions[function_id](function_id, 'immediate')
+  return functions[function_id](function_id, 'immediate')
 end, "protected")
 -- functions[43] = function() -- \pdfvariable
 --   local name = token.scan_string()
diff --git a/luametalatex-init.lua b/luametalatex-init.lua
index 6e140a8..7711f0c 100644
--- a/luametalatex-init.lua
+++ b/luametalatex-init.lua
@@ -98,7 +98,8 @@ callback_register('handle_error_hook', function()
     if line == "" then return 3 end
     local first = line:sub(1,1):upper()
     if first == 'H' then
-      texio.write(tex.gethelptext())
+      texio.write(tex.gethelptext() or "Sorry, I don't know how to help in this situation.\n\z
+        Maybe you should try asking a human?")
     elseif first == 'I' then
       line = line:sub(2)
       tex.runtoks(function()
diff --git a/luametalatex-nodewriter.lua b/luametalatex-nodewriter.lua
index b6ce05d..07fd02d 100644
--- a/luametalatex-nodewriter.lua
+++ b/luametalatex-nodewriter.lua
@@ -31,6 +31,8 @@ local rangedimensions = direct.rangedimensions
 local traverse_id = direct.traverse_id
 local getdata = direct.getdata
 
+local get_whatsit_handler = require'luametalatex-whatsits'.handler
+
 local dir_id = node.id'dir'
 
 local function doublekeyed(t, id2name, name2id, index)
@@ -470,9 +472,9 @@ function nodehandler.glyph(p, n, x, y, ...)
   p.pos.x = p.pos.x + math.floor(getwidth(n)*(1+getexpansion(n)/1000000)+.5)
 end
 function nodehandler.whatsit(p, n, ...) -- Whatsit?
-  local prop = properties[n]-- or node.getproperty(n)
-  if prop and prop.handle then
-    prop:handle(p, n, ...)
+  local handler, prop = get_whatsit_handler(n)
+  if handler then
+    handler(prop, p, n, ...)
   else
     write("Invalid whatsit found (missing handler).")
   end
diff --git a/luametalatex-whatsits.lua b/luametalatex-whatsits.lua
new file mode 100644
index 0000000..2bed3de
--- /dev/null
+++ b/luametalatex-whatsits.lua
@@ -0,0 +1,96 @@
+local whatsit_id = node.id'whatsit'
+local whatsits = {
+  [0] = "open",
+        "write",
+        "close",
+        "special",
+        nil,
+        nil,
+        "save_pos",
+        "late_lua",
+        "user_defined",
+        nil,
+        nil,
+        nil,
+        nil,
+        nil,
+        nil,
+        nil,
+        "pdf_literal",
+        "pdf_refobj",
+        "pdf_annot",
+        "pdf_start_link",
+        "pdf_end_link",
+        "pdf_dest",
+        "pdf_action",
+        "pdf_thread",
+        "pdf_start_thread",
+        "pdf_end_thread",
+        "pdf_thread_data",
+        "pdf_link_data",
+        "pdf_colorstack",
+        "pdf_setmatrix",
+        "pdf_save",
+        "pdf_restore",
+}
+local whatsithandler = {}
+-- for i = 0,#whatsits do -- #whatsits isn't guaranteed to work because of the nil entries
+for i = 0,31 do
+  local v = whatsits[i]
+  if v then
+    whatsits[v] = i
+  end
+end
+function node.whatsits() return whatsits end
+function node.subtype(s) return type(s) == "string" and whatsits[s] or nil end
+
+local direct = node.direct
+local getsubtype = direct.getsubtype
+local properties = direct.get_properties_table()
+local tonode, todirect = direct.tonode, direct.todirect
+
+local function get_handler(n, subtype)
+  local props = properties[n]
+  return props and props.handle or whatsithandler[subtype or getsubtype(n)], props
+end
+local function new(name, handler)
+  assert(type(name) == 'string')
+  local subtype = whatsits[name]
+  if subtype then
+    if whatsithandler[subtype] then
+      texio.write_nl'WARNING: Overwriting default whatsit handler'
+    end
+  else
+    subtype = #whatsits + 1
+    whatsits[subtype] = name
+    whatsits[name] = subtype
+  end
+  whatsithandler[subtype] = handler
+  return subtype
+end
+
+-- TODO: Some fields might expect different values
+local function setwhatsitfield(n, name, value)
+  local props = properties[n]
+  if not props then
+    props = {}
+    properties[n] = props
+  end
+  props[name] = value
+end
+direct.setwhatsitfield = setwhatsitfield
+
+local function getwhatsitfield(n, name)
+  local props = properties[n]
+  return props and props[name]
+end
+direct.getwhatsitfield = getwhatsitfield
+
+-- TODO: Some fields might be nodes and therefore have to be converted
+function node.setwhatsitfield(n, ...) return setwhatsitfield(todirect(n), ...) end
+function node.getwhatsitfield(n, ...) return getwhatsitfield(todirect(n), ...) end
+
+return {
+  handler = get_handler,
+  new = new,
+}