diff --git a/backend/src/model/register.rs b/backend/src/model/register.rs
index 4ec2634..b3c8888 100644
--- a/backend/src/model/register.rs
+++ b/backend/src/model/register.rs
@@ -88,7 +88,7 @@ impl RegisterCreate {
         } else {
             let res = sqlx::query!(
                 "SELECT MAX(register_code) AS max FROM register
-                WHERE register_course_id = ?",
+                WHERE register_course_id = '?'",
                 course_id
             )
             .fetch_one(db)