diff --git a/openedx_lti_tool_plugin/deep_linking/forms.py b/openedx_lti_tool_plugin/deep_linking/forms.py
index bb5a6c9..c37a429 100644
--- a/openedx_lti_tool_plugin/deep_linking/forms.py
+++ b/openedx_lti_tool_plugin/deep_linking/forms.py
@@ -70,7 +70,7 @@ def get_content_items_choice(self, course, request: HttpRequest) -> Tuple[str, s
             course.learning_context.title,
         )
 
-    def get_deep_link_resources(self) -> Optional[Set[DeepLinkResource]]:
+    def get_deep_link_resources(self) -> Set[Optional[DeepLinkResource]]:
         """Get set of `DeepLinkResource` from cleaned `content_items` field.
 
         Returns: