diff --git a/setup.py b/setup.py
index f5f7f71..766f1b6 100755
--- a/setup.py
+++ b/setup.py
@@ -49,7 +49,7 @@ except:
 
 setuptools.setup(
         name='warcprox',
-        version='2.2b1.dev103',
+        version='2.2b1.dev104',
         description='WARC writing MITM HTTP/S proxy',
         url='https://github.com/internetarchive/warcprox',
         author='Noah Levitt',
diff --git a/warcprox/writerthread.py b/warcprox/writerthread.py
index f8cdd7a..e422a65 100644
--- a/warcprox/writerthread.py
+++ b/warcprox/writerthread.py
@@ -41,7 +41,7 @@ class WarcWriterThread(threading.Thread):
 
     def __init__(
             self, name='WarcWriterThread', recorded_url_q=None,
-            writer_pool=None, dedup_db=None, listeners=None,
+            writer_pool=None, dedup_db=None, listeners=[],
             options=warcprox.Options()):
         """recorded_url_q is a queue.Queue of warcprox.warcprox.RecordedUrl."""
         threading.Thread.__init__(self, name=name)