diff --git a/tests/conftest.py b/tests/conftest.py
index 5b44847168396..17dc87b861ed8 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1018,8 +1018,7 @@ class VllmRunner:
 
         perplexities = []
         for output in outputs:
-            assert isinstance(output, TokensTextLogprobsPromptLogprobs)
-
+            output = cast(TokensTextLogprobsPromptLogprobs, output)
             token_datas = cast(list[LogprobsOnePosition | None], output[3])
             assert token_datas[0] is None
             token_log_probs = []