diff --git a/loopy/type_inference.py b/loopy/type_inference.py
index 96e436966..bd4271d3d 100644
--- a/loopy/type_inference.py
+++ b/loopy/type_inference.py
@@ -613,6 +613,7 @@ def __init__(self, kernel, callables, new_assignments=None):
         self.kernel = kernel
         self.callables = callables
         self.new_assignments = new_assignments
+        self.symbols_with_unknown_types = set()
         CombineMapper.__init__(self)
 
     # {{{ disabled interface