diff --git a/services/convert/convert.go b/services/convert/convert.go
index 744153d8c3..259ce13dd9 100644
--- a/services/convert/convert.go
+++ b/services/convert/convert.go
@@ -596,6 +596,9 @@ func ListActionWorkflows(ctx context.Context, gitrepo *git.Repository, repo *rep
 }
 
 func GetActionWorkflow(ctx context.Context, gitrepo *git.Repository, repo *repo_model.Repository, workflowID string) (*api.ActionWorkflow, error) {
+	if repo.DefaultBranch == "" {
+		return nil, util.NewNotExistErrorf("workflow %q not found", workflowID)
+	}
 	defaultBranchCommit, err := gitrepo.GetBranchCommit(repo.DefaultBranch)
 	if err != nil {
 		return nil, err