diff --git a/loopy/type_inference.py b/loopy/type_inference.py
index 010a0658f71bcfcb037a81c6b61fd9417fc98b75..714c830b45f71ab9504f19bc7b0cc8d56dde4e29 100644
--- a/loopy/type_inference.py
+++ b/loopy/type_inference.py
@@ -118,6 +118,11 @@ class TypeInferenceMapper(CombineMapper):
                 0 <= len(dtype_set) <= 1
                 for dtype_set in dtype_sets)
 
+        # don't infer if dtype of a dependency is not known
+        for dtype_set in dtype_sets:
+            if dtype_set == []:
+                return []
+
         from pytools import is_single_valued
 
         dtypes = [dtype