diff --git a/metagpt/rag/engines/simple.py b/metagpt/rag/engines/simple.py
index 1b8a63434..e036f6aa9 100644
--- a/metagpt/rag/engines/simple.py
+++ b/metagpt/rag/engines/simple.py
@@ -94,14 +94,19 @@ class SimpleEngine(RetrieverQueryEngine):
 
     def add_docs(self, input_files: list[str]):
         """Add docs to retriever. retriever must has add_nodes func."""
-        if not isinstance(self.retriever, ModifiableRAGRetriever):
-            raise TypeError(f"must be inplement to add_docs: {type(self.retriever)}")
+        self._ensure_retriever_modifiable()
 
         documents = SimpleDirectoryReader(input_files=input_files).load_data()
         nodes = self.index.service_context.node_parser.get_nodes_from_documents(documents)
         self.retriever.add_nodes(nodes)
 
-    def add_objs(self, obj_list: list[RAGObject]):
+    def add_objs(self, objs: list[RAGObject]):
         """Adds objects to the retriever, storing each object's original form in metadata for future reference."""
-        nodes = [TextNode(text=obj.rag_key(), metadata={"obj": obj}) for obj in obj_list]
+        self._ensure_retriever_modifiable()
+
+        nodes = [TextNode(text=obj.rag_key(), metadata={"obj": obj}) for obj in objs]
         self.retriever.add_nodes(nodes)
+
+    def _ensure_retriever_modifiable(self):
+        if not isinstance(self.retriever, ModifiableRAGRetriever):
+            raise TypeError(f"the retriever is not modifiable: {type(self.retriever)}")