diff --git a/src/GF/Speech/SRG.hs b/src/GF/Speech/SRG.hs
index 995eb3b2e..8c3c5e02c 100644
--- a/src/GF/Speech/SRG.hs
+++ b/src/GF/Speech/SRG.hs
@@ -185,7 +185,9 @@ ungroupTokens = joinRE . mapRE (symbol (RESymbol . Cat) (REConcat . map (RESymbo
 --
 
 lookupFM_ :: (Ord key, Show key) => Map key elt -> key -> elt
-lookupFM_ fm k = Map.findWithDefault (error $ "Key not found: " ++ show k) k fm
+lookupFM_ fm k = Map.findWithDefault err k fm
+  where err = error $ "Key not found: " ++ show k
+                      ++ "\namong " ++ show (Map.keys fm)
 
 prtS :: Print a => a -> ShowS
 prtS = showString . prt
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