diff --git a/docs/API.md b/docs/API.md
index d4281efe2..2b7750100 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -1208,9 +1208,15 @@ Omitting params:
 Trying to add a user over non-localhost:
 ```json
 {
-  "result": 0,
   "id": 1,
-  "jsonrpc": "2.0"
+  "jsonrpc": "2.0",
+  "error": {
+    "code": -32603,
+    "data": {
+      "remote_ip": "10.0.0.1"
+    },
+    "message": "Call to \"add_api_user\" method not permitted from a remote IP"
+  }
 }
 ```
 
diff --git a/lib/Zonemaster/Backend/RPCAPI.pm b/lib/Zonemaster/Backend/RPCAPI.pm
index 76e20add2..5f3c178a9 100644
--- a/lib/Zonemaster/Backend/RPCAPI.pm
+++ b/lib/Zonemaster/Backend/RPCAPI.pm
@@ -575,6 +575,12 @@ sub add_api_user {
         if ( $allow ) {
             $result = 1 if ( $self->{db}->add_api_user( $params->{username}, $params->{api_key} ) eq '1' );
         }
+        else {
+            die Zonemaster::Backend::Error::PermissionDenied->new(
+                message => 'Call to "add_api_user" method not permitted from a remote IP',
+                data => { remote_ip => $remote_ip }
+            );
+        }
     };
     if ($@) {
         handle_exception( $@ );