diff --git a/litellm/proxy/rag_endpoints/endpoints.py b/litellm/proxy/rag_endpoints/endpoints.py
index f2cef9b80a5..8e247b5228d 100644
--- a/litellm/proxy/rag_endpoints/endpoints.py
+++ b/litellm/proxy/rag_endpoints/endpoints.py
@@ -13,6 +13,7 @@ from typing import TYPE_CHECKING, Any, Final
 import orjson
 from fastapi import APIRouter, Depends, HTTPException, Request, Response, status
 from fastapi.responses import ORJSONResponse, StreamingResponse
+from typing_extensions import ReadOnly, TypedDict
 
 import litellm
 from litellm._logging import verbose_proxy_logger
@@ -46,6 +47,15 @@ if TYPE_CHECKING:
 router: Final = APIRouter()
 
 
+class _RAGIngestErrorDetail(TypedDict):
+    error: ReadOnly[str]
+
+
+def _rag_ingest_bad_request(message: str) -> HTTPException:
+    detail: Final[_RAGIngestErrorDetail] = {"error": message}
+    return HTTPException(status_code=400, detail=detail)
+
+
 def _raise_vector_store_scan_depth_exceeded() -> None:
     raise HTTPException(
         status_code=400,
@@ -430,14 +440,13 @@ async def parse_rag_ingest_request(
                 )
 
         provider: Final = vector_store_opts.get("custom_llm_provider", "openai")
-        if isinstance(provider, str):
-            try:
-                get_ingestion_class(provider)
-            except ValueError as error:
-                raise HTTPException(
-                    status_code=400,
-                    detail={"error": str(error)},
-                ) from error
+        if not isinstance(provider, str):
+            raise _rag_ingest_bad_request("custom_llm_provider must be a string")
+
+        try:
+            get_ingestion_class(provider)
+        except ValueError as error:
+            raise _rag_ingest_bad_request(str(error)) from error
 
     return ingest_options, secured_file_data, file_url, file_id
 
diff --git a/tests/test_litellm/proxy/rag_endpoints/test_rag_endpoints.py b/tests/test_litellm/proxy/rag_endpoints/test_rag_endpoints.py
index 14473fecf00..ee81aae8b78 100644
--- a/tests/test_litellm/proxy/rag_endpoints/test_rag_endpoints.py
+++ b/tests/test_litellm/proxy/rag_endpoints/test_rag_endpoints.py
@@ -7,12 +7,11 @@ Covers:
 
 import io
 from typing import Final
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 from fastapi.testclient import TestClient
 
-
 from litellm.proxy._types import LitellmUserRoles, UserAPIKeyAuth
 from litellm.proxy.auth.user_api_key_auth import user_api_key_auth
 from litellm.proxy.proxy_server import app
@@ -77,6 +76,33 @@ def test_rag_ingest_rejects_unsupported_provider_before_execution(
     mock_aingest.assert_not_awaited()
 
 
+def test_rag_ingest_rejects_non_string_provider_before_execution(
+    client_internal_user: TestClient,
+) -> None:
+    with (
+        patch(
+            "litellm.proxy.rag_endpoints.endpoints.litellm.aingest",
+            new_callable=AsyncMock,
+        ) as mock_aingest,
+        patch("litellm.proxy.proxy_server.prisma_client", None),
+    ):
+        response: Final = client_internal_user.post(
+            "/v1/rag/ingest",
+            json={
+                "file_id": "file-test",
+                "ingest_options": {
+                    "vector_store": {"custom_llm_provider": {"provider": "milvus"}}
+                },
+            },
+        )
+
+    assert response.status_code == 400
+    assert response.json() == {
+        "detail": {"error": "custom_llm_provider must be a string"}
+    }
+    mock_aingest.assert_not_awaited()
+
+
 def test_internal_user_viewer_rag_ingest_without_vector_store_id_rejected(
     client_internal_user_viewer,
 ):