diff --git a/app/controllers/api/proxy.php b/app/controllers/api/proxy.php
index 71125d2c87..c3a9fbe3ee 100644
--- a/app/controllers/api/proxy.php
+++ b/app/controllers/api/proxy.php
@@ -47,6 +47,12 @@ App::post('/v1/proxy/rules')
         if ($domain === $mainDomain) {
             throw new Exception(Exception::GENERAL_ARGUMENT_INVALID, 'You cannot assign your main domain to specific resource. Please use subdomain or a different domain.');
         }
+
+        $functionsDomain = App::getEnv('_APP_DOMAIN_FUNCTIONS', '');
+        if (str_ends_with($functionsDomain, $domain)) {
+            throw new Exception(Exception::GENERAL_ARGUMENT_INVALID, 'You cannot assign your functions domain or it\'s subdomain to specific resource. Please use different domain.');
+        }
+
         if ($domain === 'localhost' || $domain === APP_HOSTNAME_INTERNAL) {
             throw new Exception(Exception::GENERAL_ARGUMENT_INVALID, 'This domain name is not allowed. Please pick another one.');
         }