diff --git a/vllm/multimodal/processing.py b/vllm/multimodal/processing.py
index a0993f377dea..aeb8ec453c13 100644
--- a/vllm/multimodal/processing.py
+++ b/vllm/multimodal/processing.py
@@ -206,6 +206,7 @@ def get_replacement(self, item_idx: int) -> _BoundPromptReplacementGroup:
 
         if not isinstance(replacement, PromptReplacementDetails):
             replacement = PromptReplacementDetails.from_seq(replacement)
+            assert isinstance(replacement, PromptReplacementDetails)
 
         bound_full = _BoundPromptSequence.from_seq(self.tokenizer,
                                                    replacement.full)