diff --git a/builtin/commit.c b/builtin/commit.c
index d75b380..11c4740 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1728,7 +1728,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 				allow_fast_forward = 0;
 		}
 		if (allow_fast_forward)
-			parents = reduce_heads(parents);
+			reduce_heads_replace(&parents);
 	} else {
 		if (!reflog_msg)
 			reflog_msg = (whence == FROM_CHERRY_PICK)