diff --git a/src/Issue/IssueField.php b/src/Issue/IssueField.php
index d40dc901..9f2910f5 100644
--- a/src/Issue/IssueField.php
+++ b/src/Issue/IssueField.php
@@ -411,7 +411,7 @@ public function setParentKeyOrId($keyOrId)
         return $this;
     }
 
-    public function setParent(Issue $parent)
+    public function setParent(?Issue $parent)
     {
         $this->parent = $parent;
     }