diff --git a/hgext/convert/subversion.py b/hgext/convert/subversion.py
--- a/hgext/convert/subversion.py
+++ b/hgext/convert/subversion.py
@@ -439,6 +439,8 @@ class svn_source(converter_source):
             if revnum < stop:
                 stop = revnum + 1
             self._fetch_revisions(revnum, stop)
+            if rev not in self.commits:
+                raise util.Abort(_('svn: revision %s not found') % revnum)
         commit = self.commits[rev]
         # caller caches the result, so free it here to release memory
         del self.commits[rev]