diff --git a/artiq/coredevice/comm_dummy.py b/artiq/coredevice/comm_dummy.py
index b9eb648fe..84aafb666 100644
--- a/artiq/coredevice/comm_dummy.py
+++ b/artiq/coredevice/comm_dummy.py
@@ -6,8 +6,7 @@ from artiq.coredevice.runtime import LinkInterface
 
 
 class _RuntimeEnvironment(LinkInterface):
-    def __init__(self, ref_period):
-        self.internal_ref_period = ref_period
+    def __init__(self):
         self.warmup_time = 1*ms
 
     def emit_object(self):
@@ -16,7 +15,7 @@ class _RuntimeEnvironment(LinkInterface):
 
 class Comm(AutoDB):
     def get_runtime_env(self):
-        return _RuntimeEnvironment(1*ns)
+        return _RuntimeEnvironment()
 
     def switch_clock(self, external):
         pass
diff --git a/artiq/test/transforms.py b/artiq/test/transforms.py
index c7893f236..b9ee5e30d 100644
--- a/artiq/test/transforms.py
+++ b/artiq/test/transforms.py
@@ -1,6 +1,7 @@
 import unittest
 import ast
 
+from artiq import ns
 from artiq.coredevice import comm_dummy, core
 from artiq.transforms.unparse import unparse
 
@@ -39,7 +40,7 @@ def run():
 
 class OptimizeCase(unittest.TestCase):
     def test_optimize(self):
-        coredev = core.Core(comm=comm_dummy.Comm())
+        coredev = core.Core(comm=comm_dummy.Comm(), ref_period=1*ns)
         func_def = ast.parse(optimize_in).body[0]
         coredev.transform_stack(func_def, dict(), dict())
         self.assertEqual(unparse(func_def), optimize_out)