diff --git a/pillar/api/utils/__init__.py b/pillar/api/utils/__init__.py
index 81411445..bb332b27 100644
--- a/pillar/api/utils/__init__.py
+++ b/pillar/api/utils/__init__.py
@@ -122,11 +122,9 @@ def project_get_node_type(project_document, node_type_node_name):
                  if node_type['name'] == node_type_node_name), None)
 
 
-def str2id(document_id):
+def str2id(document_id: str) -> bson.ObjectId:
     """Returns the document ID as ObjectID, or raises a BadRequest exception.
 
-    :type document_id: str
-    :rtype: bson.ObjectId
     :raises: wz_exceptions.BadRequest
     """