diff --git a/test/registered/unit/hardware_backend/mlx/test_attention_patching.py b/test/registered/unit/hardware_backend/mlx/test_attention_patching.py
index 2eb416e52..e58518785 100644
--- a/test/registered/unit/hardware_backend/mlx/test_attention_patching.py
+++ b/test/registered/unit/hardware_backend/mlx/test_attention_patching.py
@@ -1520,6 +1520,7 @@ if _HAS_MLX:
     class FakeRequest:
         def __init__(self):
             self.kv = ReqKvInfo()
+            self.mamba_branching_seqlen = None
             self.inflight_middle_chunks = 0
 
     class FakeTpWorker: