diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -187,12 +187,12 @@ preferedchunksize = 4096
 
 def outdebug(ui, message):
     """debug regarding output stream (bundling)"""
-    if ui.configbool('devel', 'bundle2.debug', False):
+    if ui.configbool('devel', 'bundle2.debug'):
         ui.debug('bundle2-output: %s\n' % message)
 
 def indebug(ui, message):
     """debug on input stream (unbundling)"""
-    if ui.configbool('devel', 'bundle2.debug', False):
+    if ui.configbool('devel', 'bundle2.debug'):
         ui.debug('bundle2-input: %s\n' % message)
 
 def validateparttype(parttype):
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -59,6 +59,9 @@ coreconfigitem = getitemregister(coreite
 coreconfigitem('devel', 'all-warnings',
     default=False,
 )
+coreconfigitem('devel', 'bundle2.debug',
+    default=False,
+)
 coreconfigitem('patch', 'fuzz',
     default=2,
 )