diff --git a/src/Persistence/WorkflowSerializableInterface.php b/src/Persistence/WorkflowSerializableInterface.php
index 687071f..c4a5d23 100644
--- a/src/Persistence/WorkflowSerializableInterface.php
+++ b/src/Persistence/WorkflowSerializableInterface.php
@@ -18,12 +18,12 @@
 interface WorkflowSerializableInterface extends WorkflowAwareInterface
 {
     /**
-     * @param string $serializedWorkflow
+     * @param string|null $serializedWorkflow
      */
     public function setSerializedWorkflow($serializedWorkflow);
 
     /**
-     * @return string
+     * @return string|null
      */
     public function getSerializedWorkflow();