diff --git a/src/Database/Esqueleto/Internal/Sql.hs b/src/Database/Esqueleto/Internal/Sql.hs
index 63c801c..e13877a 100644
--- a/src/Database/Esqueleto/Internal/Sql.hs
+++ b/src/Database/Esqueleto/Internal/Sql.hs
@@ -102,6 +102,7 @@ data CompositeKeyError =
   | FoldHelpError
   | SqlCaseError
   | SqlBinOpError
+  | SqlCastAsError
   | MakeOnClauseError
   | MakeExcError
   | MakeSetError
@@ -766,13 +767,14 @@ unsafeSqlFunctionParens name arg =
           uncommas' $ map (\(ERaw p f) -> first (parensM p) (f info)) $ toArgList arg
     in (name <> parens argsTLB, argsVals)
 
-
--- | explicit cast using CAST(value as type)
+-- | (Internal) An explicit SQL type cast using CAST(value as type).
+-- See 'unsafeSqlBinOp' for warnings.
 unsafeSqlCastAs :: T.Text -> SqlExpr (Value a) -> SqlExpr (Value b)
 unsafeSqlCastAs t (ERaw p f) =
   ERaw Never $ \info ->
     let (b, v) = f info
     in ("CAST" <> parens ( parensM p b <> " AS " <> TLB.fromText t), v )
+unsafeSqlCastAs _ (ECompositeKey _) = throw (CompositeKeyErr SqlCastAsError)
 
 class UnsafeSqlFunctionArgument a where
   toArgList :: a -> [SqlExpr (Value ())]