diff --git a/routers/api/v1/repo/pull.go b/routers/api/v1/repo/pull.go
index 2426a6b3c2..2702f6864f 100644
--- a/routers/api/v1/repo/pull.go
+++ b/routers/api/v1/repo/pull.go
@@ -996,7 +996,7 @@ func MergePullRequest(ctx *context.APIContext) {
 				return
 			}
 			if strings.Contains(err.Error(), "Wrong commit ID") {
-				ctx.JSON(http.StatusConflict, err)
+				ctx.APIError(http.StatusConflict, err)
 				return
 			}
 			ctx.APIErrorInternal(err)