diff --git a/migen/test/support.py b/migen/test/support.py
index 394cc424d..51a897079 100644
--- a/migen/test/support.py
+++ b/migen/test/support.py
@@ -17,6 +17,6 @@ class SimCase:
 	def test_to_verilog(self):
 		verilog.convert(self.tb)
 
-	def run_with(self, cb, ncycles=-1):
+	def run_with(self, cb, ncycles=None):
 		self.tb.callback = cb
 		run_simulation(self.tb, ncycles=ncycles)