diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -1430,7 +1430,7 @@
 
     # By convention, ``extra['branch']`` (set by extrafn) clobbers
     # ``branch`` (used when passing ``--keepbranches``).
-    branch = repo[p1].branch()
+    branch = None
     if b'branch' in extra:
         branch = extra[b'branch']
 
diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -2445,6 +2445,9 @@
                 # necessary for memctx to register a deletion.
                 return None
 
+        if branch is None:
+            branch = self._wrappedctx.branch()
+
         return memctx(
             self._repo,
             parents,