diff --git a/src/backend/src/routers/signup.js b/src/backend/src/routers/signup.js
index 4aa7fa6c..06f7905e 100644
--- a/src/backend/src/routers/signup.js
+++ b/src/backend/src/routers/signup.js
@@ -24,6 +24,7 @@ const config = require('../config');
 const eggspress = require('../api/eggspress');
 const { Context } = require('../util/context');
 const { DB_WRITE } = require('../services/database/consts');
+const { can } = require('../util/langutil');
 
 // -----------------------------------------------------------------------//
 // POST /signup
@@ -146,7 +147,7 @@ module.exports = eggspress(['/signup'], {
     const svc_cleanEmail = req.services.get('clean-email');
     const clean_email = svc_cleanEmail.clean(req.body.email);
     
-    if ( config.blocked_email_domains ) {
+    if ( can(config.blocked_email_domains, 'iterate') ) {
         for ( const suffix of config.blocked_email_domains ) {
             if ( clean_email.endsWith(suffix) ) {
                 return res.status(400).send('This email domain is not allowed.');
diff --git a/src/backend/src/util/langutil.js b/src/backend/src/util/langutil.js
index ceea451f..443dc2f1 100644
--- a/src/backend/src/util/langutil.js
+++ b/src/backend/src/util/langutil.js
@@ -28,7 +28,22 @@ const whatis = thing => {
 
 const nou = v => v === null || v === undefined;
 
+const can = (v, ...checking) => {
+    if ( nou(v) ) return false;
+    const capabilities = {};
+    if ( v[Symbol.iterator] ) {
+        capabilities['iterate'] = true;
+    }
+    for ( const to_check of checking ) {
+        if ( ! capabilities[to_check] ) {
+            return false;
+        }
+    }
+    return true;
+}
+
 module.exports = {
     whatis,
     nou,
+    can,
 };