diff -r 44611ad4fbd9 -r f4a42bb7c2ec hgext/convert/git.py
--- a/hgext/convert/git.py	Wed Apr 06 18:19:36 2016 +0000
+++ b/hgext/convert/git.py	Mon Apr 04 15:38:48 2016 +0200
@@ -357,6 +357,8 @@
             output, status = self.gitrunlines('diff-tree', '--name-only',
                                               '--root', '-r', version,
                                               '%s^%s' % (version, i + 1), '--')
+            if status:
+                raise error.Abort(_('cannot read changes in %s') % version)
             changes = [f.rstrip('\n') for f in output]
 
         return changes