diff --git a/migen/uio/ioo.py b/migen/uio/ioo.py
index 1393d9150..e6c954b55 100644
--- a/migen/uio/ioo.py
+++ b/migen/uio/ioo.py
@@ -15,7 +15,7 @@ class UnifiedIOObject(Actor):
 		self._memories = set(v for v in self.buses.values() if isinstance(v, Memory))
 	
 	def get_fragment(self):
-		return Fragment(specials={self._memories})
+		return Fragment(specials=self._memories)
 
 (_WAIT_COMPLETE, _WAIT_POLL) = range(2)