diff --git a/src/Settings/WellKnownFiles/TH.hs b/src/Settings/WellKnownFiles/TH.hs
index 3844bf664..ada0a083e 100644
--- a/src/Settings/WellKnownFiles/TH.hs
+++ b/src/Settings/WellKnownFiles/TH.hs
@@ -110,7 +110,7 @@ mkWellKnown defLang wellKnownBase wellKnownLinks = do
     , let languages' = Set.delete defLang $ Map.keysSet languageLinks
       -> return $ defLang :| Set.toList languages'
     | otherwise
-      -> fail "default language is missing in wellKnownBase"
+      -> fail ("default language is missing in wellKnownBase" ++ "; default language is '" ++ (show defLang) ++ "' ; list of known languages is " ++ (show $ Map.keys languageLinks))
 
 
   fVar <- newName "f"