diff --git a/hgext3rd/p4fastimport/p4.py b/hgext3rd/p4fastimport/p4.py
--- a/hgext3rd/p4fastimport/p4.py
+++ b/hgext3rd/p4fastimport/p4.py
@@ -490,5 +490,6 @@
             elif info['action'] in ACTION_DELETE + ACTION_ARCHIVE:
                 r.append(fname)
             else:
-                assert False
+                msg = 'unsupported action "%s" in changelist %s for file %s'
+                raise P4Exception(msg % (info['action'], self._clnum, fname))
         return a, m, r