diff --git a/hooks/first_parent.py b/hooks/first_parent.py
index 2a38af340ef2215e0aef841d627cedd7b7952c89..edf8b8ed65343c4e7cbea99012c267a778c7bc93 100755
--- a/hooks/first_parent.py
+++ b/hooks/first_parent.py
@@ -74,10 +74,26 @@ def resolve_ref(ref: str) -> Optional[str]:
 	return proc.stdout.strip() or None
 
 
+def is_ancestor(head_ref: str, other_ref: str) -> bool:
+	"""
+	Return True if "other_ref" is an ancestor of "head_ref"
+
+	Note that "head_ref" refers to the head of a DAG graph to check, not a named reference
+	in the refs/heads/ namespace.
+	"""
+	cmd = ["git", "merge-base", "--is-ancestor", other_ref, head_ref]
+	return 0 == run(cmd).returncode
+
+
 def check(ref: str, upstream: str) -> None:
 	"""
 	Exit with 1 if "upstream" can reach first-parent commits not reachable from "ref"
+
+	Skipped if "upstream" is not an ancestor of "ref".  This would indicate rebasing which
+	should be checked separately.
 	"""
+	if not is_ancestor(ref, upstream):
+		return
 	head_commits = get_fp_commits(ref)
 	upstream_commits = get_fp_commits(upstream)
 	if not upstream_commits.difference(head_commits):