diff --git a/src/allmydata/immutable/layout.py b/src/allmydata/immutable/layout.py
index 99f5bcbea..f1e416178 100644
--- a/src/allmydata/immutable/layout.py
+++ b/src/allmydata/immutable/layout.py
@@ -4,7 +4,7 @@ from twisted.internet import defer
 from allmydata.interfaces import IStorageBucketWriter, IStorageBucketReader, \
      FileTooLargeError, HASH_SIZE
 from allmydata.util import mathutil, idlib
-from allmydata.util.assertutil import _assert, precondition
+from allmydata.util.assertutil import precondition
 from allmydata import storage
 
 class LayoutInvalid(Exception):