diff --git a/devito/ir/clusters/cluster.py b/devito/ir/clusters/cluster.py
index 887793e8e7e..69ee00c6c5b 100644
--- a/devito/ir/clusters/cluster.py
+++ b/devito/ir/clusters/cluster.py
@@ -483,7 +483,7 @@ def dtype(self):
         If two Clusters perform calculations with different precision, the
         data type with highest precision is returned.
         """
-        dtypes = {i.dtype for i in self}
+        dtypes = {i.dtype for i in self} - {None}
 
         return infer_dtype(dtypes)