diff --git a/litellm/cost_calculator.py b/litellm/cost_calculator.py
index f85ca0462da..b5c6c3a914f 100644
--- a/litellm/cost_calculator.py
+++ b/litellm/cost_calculator.py
@@ -888,7 +888,9 @@ def _select_model_name_for_cost_calc(
             base_model
             if base_model is not None
             else _priced_provider_response_model(
-                provider_response_model, completion_response_model, custom_llm_provider
+                provider_response_model,
+                completion_response_model if isinstance(completion_response_model, str) else None,
+                custom_llm_provider,
             )
         )