diff --git a/services/pull/merge.go b/services/pull/merge.go
index 3d856724c..2c4f939e0 100644
--- a/services/pull/merge.go
+++ b/services/pull/merge.go
@@ -411,7 +411,7 @@ func rawMerge(pr *models.PullRequest, doer *models.User, mergeStyle models.Merge
 	)
 
 	// Push back to upstream.
-	if err := git.NewCommand("push", "origin", baseBranch+":"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+	if err := git.NewCommand("push", "origin", baseBranch+":refs/heads/"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
 		if strings.Contains(errbuf.String(), "non-fast-forward") {
 			return "", &git.ErrPushOutOfDate{
 				StdOut: outbuf.String(),