diff --git a/merge-recursive.c b/merge-recursive.c
index b058741f00..e12d91f48a 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -3550,6 +3550,8 @@ static int merge_recursive_internal(struct merge_options *opt,
 		merged_merge_bases = make_virtual_commit(opt->repo, tree,
 							 "ancestor");
 		ancestor_name = "empty tree";
+	} else if (opt->ancestor) {
+		ancestor_name = opt->ancestor;
 	} else if (merge_bases) {
 		ancestor_name = "merged common ancestors";
 	} else {
@@ -3689,7 +3691,8 @@ int merge_recursive(struct merge_options *opt,
 {
 	int clean;
 
-	assert(opt->ancestor == NULL);
+	assert(opt->ancestor == NULL ||
+	       !strcmp(opt->ancestor, "constructed merge base"));
 
 	if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
 		return -1;
@@ -3741,6 +3744,8 @@ int merge_recursive_generic(struct merge_options *opt,
 					   oid_to_hex(merge_bases[i]));
 			commit_list_insert(base, &ca);
 		}
+		if (num_merge_bases == 1)
+			opt->ancestor = "constructed merge base";
 	}
 
 	repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);
diff --git a/t/t6047-diff3-conflict-markers.sh b/t/t6047-diff3-conflict-markers.sh
index 3fb68e0aae..860542aad0 100755
--- a/t/t6047-diff3-conflict-markers.sh
+++ b/t/t6047-diff3-conflict-markers.sh
@@ -186,4 +186,17 @@ test_expect_success 'check multiple merge bases' '
 	)
 '
 
+test_expect_success 'rebase describes fake ancestor base' '
+	test_create_repo rebase &&
+	(
+		cd rebase &&
+		test_commit base file &&
+		test_commit master file &&
+		git checkout -b side HEAD^ &&
+		test_commit side file &&
+		test_must_fail git -c merge.conflictstyle=diff3 rebase master &&
+		grep "||||||| constructed merge base" file
+	)
+'
+
 test_done