diff --git a/luamml-convert.lua b/luamml-convert.lua
index bdd4684..51996d2 100644
--- a/luamml-convert.lua
+++ b/luamml-convert.lua
@@ -273,7 +273,7 @@ local function noad_to_table(noad, sub, cur_style, joining, bin_replacements, te
     end
     if nucleus == core and #core == 1 then
       if joining and joining[0] == 'mn' and core[0] == 'mi' and (core[1] == '.' or core[1] == ',') and maybe_to_mn(noad, core)
-          or core[0] == 'mn' or text_families[core['tex:family']] then
+          or core[0] == 'mn' or text_families[core['tex:family'] or 0] then
         if joining and core[0] == joining[0] and core['tex:family'] == joining['tex:family'] then
           joining[#joining+1] = core[1]
           local cnodes = core[':nodes']
diff --git a/luamml-tex.lua b/luamml-tex.lua
index a399db4..d6cee83 100644
--- a/luamml-tex.lua
+++ b/luamml-tex.lua
@@ -24,7 +24,7 @@ local text_families_meta = {__index = function(t, fam)
       -- FIXME(?): If there is no font...
       error'Please load your fonts?!?'
     end
-    assignment = not fontdir.MathConstants
+    assignment = not (fontdir.MathConstants and next(fontdir.MathConstants))
   end
   t[fam] = assignment
   return assignment
@@ -50,6 +50,9 @@ lua.get_functions_table()[funcid] = function()
   local mapping = token.scan_string()
   if mappings[mapping] then
     register_family(fam, mappings[mapping])
+    if global_text_families[fam] == nil then
+      global_text_families[fam] = false
+    end
   else
     tex.error(string.format('Unknown font mapping %q', mapping))
   end