400 when attempting to filter on an attribute of a polymorphic relationship

src/ResourceType.php

@@ -157,9 +157,13 @@ final class ResourceType
                         $this->adapter->filterByRelationship($query, $field, function ($query) use ($relatedResource, $sub, $value, $context) {
                             $relatedResource->applyFilters($query, [($sub ?? 'id') => $value], $context);
                         });
-                    }
+
                         continue;
                     }
+
+                    throw (new BadRequestException('Cannot filter on attribute of polymorphic relationship: '.$name))
+                        ->setSourceParameter("filter[$name]");
+                }
             }
 
             throw (new BadRequestException("Invalid filter: $name"))->setSourceParameter("filter[$name]");