diff --git a/src/compiler/GF/Compile/GrammarToPGF.hs b/src/compiler/GF/Compile/GrammarToPGF.hs
index e10a2b13b..5ec7f9774 100644
--- a/src/compiler/GF/Compile/GrammarToPGF.hs
+++ b/src/compiler/GF/Compile/GrammarToPGF.hs
@@ -261,7 +261,7 @@ genCncFuns gr am cm seqs0 cdefs fid_cnt cnccats =
     mkFId (_,cat) fid0 =
       case Map.lookup (i2i cat) cnccats of
         Just (C.CncCat s e _) -> s+fid0
-        Nothing               -> error "GrammarToPGF.mkFId failed"
+        Nothing               -> error ("GrammarToPGF.mkFId: missing category "++showIdent cat)
 
     mkCtxt lindefs (_,cat) =
       case Map.lookup (i2i cat) cnccats of