diff --git a/pytools/persistent_dict.py b/pytools/persistent_dict.py
index e362d2e47fa2ec17d57f7c66af09712b45b88b00..5ca6b91c5fd74b7360dba0b8f4f8b95e392bee51 100644
--- a/pytools/persistent_dict.py
+++ b/pytools/persistent_dict.py
@@ -226,7 +226,7 @@ class KeyBuilder(object):
         key_hash.update("<None>".encode('utf8'))
 
     def update_for_dtype(self, key_hash, key):
-        return key.str.encode("utf8")
+        key_hash.update(key.str.encode('utf8'))
 
     # }}}