diff --git a/vllm/engine/arg_utils.py b/vllm/engine/arg_utils.py
index 757023e12d439..2867532756450 100644
--- a/vllm/engine/arg_utils.py
+++ b/vllm/engine/arg_utils.py
@@ -1649,7 +1649,13 @@ class EngineArgs:
                     "attention_backend and attention_config.backend "
                     "are mutually exclusive"
                 )
-            attention_config.backend = self.attention_backend
+            # Convert string to enum if needed (CLI parsing returns a string)
+            if isinstance(self.attention_backend, str):
+                attention_config.backend = AttentionBackendEnum[
+                    self.attention_backend.upper()
+                ]
+            else:
+                attention_config.backend = self.attention_backend
 
         load_config = self.create_load_config()