diff --git a/vertexai/rag/utils/_gapic_utils.py b/vertexai/rag/utils/_gapic_utils.py
index ea140a7c88..83b63cc376 100644
--- a/vertexai/rag/utils/_gapic_utils.py
+++ b/vertexai/rag/utils/_gapic_utils.py
@@ -702,7 +702,7 @@ def set_backend_config(
     if backend_config.vector_db is not None:
         vector_config = backend_config.vector_db
         if isinstance(vector_config, RagManagedDb):
-            rag_corpus.vector_db_config.rag_managed_db.CopyFrom(
+            rag_corpus.vector_db_config.rag_managed_db = (
                 GapicRagVectorDbConfig.RagManagedDb()
             )
         elif isinstance(vector_config, VertexVectorSearch):