diff --git a/artiq/protocols/pyon.py b/artiq/protocols/pyon.py
index 13161094e..ff793945d 100644
--- a/artiq/protocols/pyon.py
+++ b/artiq/protocols/pyon.py
@@ -153,7 +153,7 @@ def _npscalar(ty, data):
 
 
 _eval_dict = {
-    "__builtins__": None,
+    "__builtins__": {},
 
     "null": None,
     "false": False,