diff --git a/src/allmydata/dirnode.py b/src/allmydata/dirnode.py
index 59ebd73ba..6f052c3c7 100644
--- a/src/allmydata/dirnode.py
+++ b/src/allmydata/dirnode.py
@@ -554,7 +554,7 @@ class DirectoryNode(object):
         d = self.get_child_and_metadata(childnamex)
         return d
 
-    def set_uri(self, namex, writecap, readcap, metadata=None, overwrite=True):
+    def set_uri(self, namex, writecap, readcap=None, metadata=None, overwrite=True):
         precondition(isinstance(writecap, (str,type(None))), writecap)
         precondition(isinstance(readcap, (str,type(None))), readcap)