diff --git a/loopy/type_inference.py b/loopy/type_inference.py
index fcf8f965b68fd258b0c0f1eae94ec84a39a5b7ee..010a0658f71bcfcb037a81c6b61fd9417fc98b75 100644
--- a/loopy/type_inference.py
+++ b/loopy/type_inference.py
@@ -337,7 +337,7 @@ class TypeInferenceMapper(CombineMapper):
         if fields is None:
             raise LoopyError("cannot look up attribute '%s' in "
                     "non-aggregate expression '%s'"
-                    % (expr.aggregate, expr.name))
+                    % (expr.name, expr.aggregate))
 
         try:
             field = fields[expr.name]