diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
--- a/mercurial/changegroup.py
+++ b/mercurial/changegroup.py
@@ -322,19 +322,19 @@ class cg1unpacker(object):
         changesets = files = revisions = 0
 
         tr = repo.transaction("\n".join([srctype, util.hidepassword(url)]))
-        # The transaction could have been created before and already
-        # carries source information. In this case we use the top
-        # level data. We overwrite the argument because we need to use
-        # the top level value (if they exist) in this function.
-        srctype = tr.hookargs.setdefault('source', srctype)
-        url = tr.hookargs.setdefault('url', url)
+        try:
+            # The transaction could have been created before and already
+            # carries source information. In this case we use the top
+            # level data. We overwrite the argument because we need to use
+            # the top level value (if they exist) in this function.
+            srctype = tr.hookargs.setdefault('source', srctype)
+            url = tr.hookargs.setdefault('url', url)
 
-        # write changelog data to temp files so concurrent readers will not see
-        # inconsistent view
-        cl = repo.changelog
-        cl.delayupdate(tr)
-        oldheads = cl.heads()
-        try:
+            # write changelog data to temp files so concurrent readers
+            # will not see an inconsistent view
+            cl = repo.changelog
+            cl.delayupdate(tr)
+            oldheads = cl.heads()
             repo.hook('prechangegroup', throw=True, **tr.hookargs)
 
             trp = weakref.proxy(tr)