diff --git a/lib/Language/Ask/ChkRaw.hs b/lib/Language/Ask/ChkRaw.hs
index e243f7a..f6c0989 100644
--- a/lib/Language/Ask/ChkRaw.hs
+++ b/lib/Language/Ask/ChkRaw.hs
@@ -793,6 +793,7 @@ chkParse (Make Pse (ParseProb c sm) m () ss (ls, rs)) = do
            ns :-/ (mempty ::- Make Pse (ParseProb c Nothing) (Stub True) (Need, False)
                   ([] :-/ Stop) ([], []))
            :-\ xs)
+      subs _ _ = gripe $ Bland "There's something I don't understand about this parse tree."
   ps <- foldMap prods <$> gamma
   case m of
     Stub b -> case ss of
diff --git a/lib/Language/Ask/Context.hs b/lib/Language/Ask/Context.hs
index abcb084..8f7830c 100644
--- a/lib/Language/Ask/Context.hs
+++ b/lib/Language/Ask/Context.hs
@@ -102,6 +102,7 @@ data Gripe
   | ParseNotTheWanted Con
   | ParseNoMake String
   | ParseStubSub
+  | Bland String
   | FAIL
   deriving Show
 
diff --git a/lib/Language/Ask/Printing.hs b/lib/Language/Ask/Printing.hs
index 53760b3..33caf7b 100644
--- a/lib/Language/Ask/Printing.hs
+++ b/lib/Language/Ask/Printing.hs
@@ -222,6 +222,7 @@ ppGripe (ParseNotTheWanted c) = return $ concat
 ppGripe (ParseNoMake s) = return $ concat
   ["This doesn't make ", s, "."]
 ppGripe ParseStubSub = return "Don't give subtrees until you've chosen a production."
+ppGripe (Bland x) = return x
 
 ppGripe FAIL = return $
   "It went wrong but I've forgotten how. Please ask a human for help."