diff --git a/src/GF/API/GrammarToHaskell.hs b/src/GF/API/GrammarToHaskell.hs
index eae98d3f4..c57cfed42 100644
--- a/src/GF/API/GrammarToHaskell.hs
+++ b/src/GF/API/GrammarToHaskell.hs
@@ -248,6 +248,7 @@ updateSkeleton cat skel rule =
  case skel of
    (cat0,rules):rr | cat0 == cat -> (cat0, rule:rules) : rr
    (cat0,rules):rr               -> (cat0, rules) : updateSkeleton cat rr rule
+   _ -> error $ cat ++ ": updating empty skeleton with" +++ show rule
 
 isListCat :: (OIdent, [(OIdent, [OIdent])]) -> Bool
 isListCat (cat,rules) = "List" `isPrefixOf` cat && length rules == 2
@@ -267,4 +268,4 @@ isConsFun f = "Cons" `isPrefixOf` f
 
 baseSize :: (OIdent, [(OIdent, [OIdent])]) -> Int
 baseSize (_,rules) = length bs
-    where Just (_,bs) = find (("Base" `isPrefixOf`) . fst) rules
\ No newline at end of file
+    where Just (_,bs) = find (("Base" `isPrefixOf`) . fst) rules