diff --git a/src/backend/src/routers/signup.js b/src/backend/src/routers/signup.js
index 0e010b9f0..d70d3d637 100644
--- a/src/backend/src/routers/signup.js
+++ b/src/backend/src/routers/signup.js
@@ -189,8 +189,8 @@ module.exports = eggspress(['/signup'], {
     const svc_cleanEmail = req.services.get('clean-email');
     const clean_email = svc_cleanEmail.clean(req.body.email);
     
-    if ( ! await svc_cleanEmail.validate(clean_email) ) {
-        return res.status(400).send('This email domain is not allowed');
+    if (clean_email !== '' && ! await svc_cleanEmail.validate(clean_email) ) {
+        return res.status(400).send('This email is not allowed.');
     }
 
     // duplicate username check