diff --git a/artiq/compiler/embedding.py b/artiq/compiler/embedding.py
index 1476dce4a..0b903dbe0 100644
--- a/artiq/compiler/embedding.py
+++ b/artiq/compiler/embedding.py
@@ -105,7 +105,8 @@ class ASTSynthesizer:
             if typ in self.type_map:
                 instance_type, constructor_type = self.type_map[typ]
             else:
-                instance_type = types.TInstance("{}.{}".format(typ.__module__, typ.__qualname__))
+                instance_type = types.TInstance("{}.{}".format(typ.__module__, typ.__qualname__),
+                                                OrderedDict())
                 instance_type.attributes['__objectid__'] = builtins.TInt(types.TValue(32))
 
                 constructor_type = types.TConstructor(instance_type)
diff --git a/artiq/compiler/types.py b/artiq/compiler/types.py
index 32501d961..73d53db52 100644
--- a/artiq/compiler/types.py
+++ b/artiq/compiler/types.py
@@ -359,7 +359,8 @@ class TInstance(TMono):
         was created
     """
 
-    def __init__(self, name, attributes=OrderedDict()):
+    def __init__(self, name, attributes):
+        assert isinstance(attributes, OrderedDict)
         super().__init__(name)
         self.attributes = attributes