diff --git a/loopy/type_inference.py b/loopy/type_inference.py
index 78d817ce73724d90a6cc6f380b24290971f6c1e7..409cbbc5ebd5feb13b04eeba1671f639663bfcf1 100644
--- a/loopy/type_inference.py
+++ b/loopy/type_inference.py
@@ -232,7 +232,8 @@ class TypeInferenceMapper(CombineMapper):
             # Codegen for complex types depends on exactly correct types.
             # Refuse temptation to guess.
             raise TypeInferenceFailure("Complex constant '%s' needs to "
-                    "be sized for type inference " % expr)
+                    "be sized (i.e. as numpy.complex64/128) for type inference "
+                    % expr)
         else:
             raise TypeInferenceFailure("Cannot deduce type of constant '%s'" % expr)