diff --git a/src/ORM.php b/src/ORM.php
index 9436d9f56..a2ac2e37d 100644
--- a/src/ORM.php
+++ b/src/ORM.php
@@ -92,6 +92,10 @@ public function resolveRole($entity): string
             $entity = $class;
         }
 
+        if (is_null($this->schema)) {
+            throw new ORMException('ORM Schema should be defined');
+        }
+
         return $this->schema->resolveAlias($entity);
     }