diff --git a/app/Interpreter.hs b/app/Interpreter.hs
index 1ef39f9..f36ef0b 100644
--- a/app/Interpreter.hs
+++ b/app/Interpreter.hs
@@ -22,7 +22,7 @@ import Env (PrlgEnv)
 
 --import Data.Function
 import qualified Data.Map as M
-import IR (Id(..))
+import IR (Id(..), StrTable(..))
 
 prove :: Code -> PrlgEnv (Either String Bool)
 prove g = do
@@ -74,7 +74,10 @@ proveStep = St.get >>= go
       d <- St.gets defs
       case d M.!? fn of
         Just d -> cont d
-        _ -> ifail $ "no definition: " ++ show fn
+        _ -> do
+          StrTable _ _ itos <- St.gets strtable
+          ifail $
+            "no definition: " ++ (itos M.! str fn) ++ "/" ++ show (arity fn)
     {- Unification -}
     go i@Interp {cur = cur@Cho {hed = U h:hs, gol = U g:gs, heap = heap}} =
       unify h g