diff --git a/libs/langchain/langchain/retrievers/parent_document_retriever.py b/libs/langchain/langchain/retrievers/parent_document_retriever.py
index f56b8929032..6757ba3ddb3 100644
--- a/libs/langchain/langchain/retrievers/parent_document_retriever.py
+++ b/libs/langchain/langchain/retrievers/parent_document_retriever.py
@@ -97,7 +97,7 @@ class ParentDocumentRetriever(BaseRetriever):
     def add_documents(
         self,
         documents: List[Document],
-        ids: Optional[List[str]],
+        ids: Optional[List[str]] = None,
         add_to_docstore: bool = True,
     ) -> None:
         """Adds documents to the docstore and vectorstores.