diff --git a/hgext/convert/cvsps.py b/hgext/convert/cvsps.py
--- a/hgext/convert/cvsps.py
+++ b/hgext/convert/cvsps.py
@@ -567,7 +567,10 @@
     mindate = {}
     for e in log:
         if e.commitid:
-            mindate[e.commitid] = min(e.date, mindate.get(e.commitid))
+            if mindate.get(e.commitid) is None:
+                mindate[e.commitid] = None
+            else:
+                mindate[e.commitid] = min(e.date, mindate.get(e.commitid))
 
     # Merge changesets
     log.sort(key=lambda x: (mindate.get(x.commitid), x.commitid, x.comment,