--- a/mercurial/metadata.py	Wed Aug 28 23:25:26 2024 +0200
+++ b/mercurial/metadata.py	Fri Sep 06 02:12:19 2024 +0200
@@ -243,7 +243,7 @@
         return _process_linear(p1, ctx)
     elif p1.rev() == nullrev and p2.rev() != nullrev:
         # In the wild, one can encounter changeset where p1 is null but p2 is not
-        return _process_linear(p1, ctx, parent=2)
+        return _process_linear(p2, ctx, parent=2)
     elif p1.rev() == p2.rev():
         # In the wild, one can encounter such "non-merge"
         return _process_linear(p1, ctx)