diff --git a/llama_toolchain/safety/shields/llama_guard.py b/llama_toolchain/safety/shields/llama_guard.py
index d6154f027..5234c8e1f 100644
--- a/llama_toolchain/safety/shields/llama_guard.py
+++ b/llama_toolchain/safety/shields/llama_guard.py
@@ -111,7 +111,7 @@ class LlamaGuardShield(ShieldBase):
     def instance(
         on_violation_action=OnViolationAction.RAISE,
         model_dir: str = None,
-        excluded_categories: List[str] = [],
+        excluded_categories: List[str] = None,
         disable_input_check: bool = False,
         disable_output_check: bool = False,
     ) -> "LlamaGuardShield":
@@ -130,7 +130,7 @@ class LlamaGuardShield(ShieldBase):
         self,
         on_violation_action: OnViolationAction = OnViolationAction.RAISE,
         model_dir: str = None,
-        excluded_categories: List[str] = [],
+        excluded_categories: List[str] = None,
         disable_input_check: bool = False,
         disable_output_check: bool = False,
     ):
@@ -140,6 +140,9 @@ class LlamaGuardShield(ShieldBase):
 
         assert model_dir is not None, "Llama Guard model_dir is None"
 
+        if excluded_categories is None:
+            excluded_categories = []
+
         assert len(excluded_categories) == 0 or all(
             x in SAFETY_CATEGORIES_TO_CODE_MAP.values() for x in excluded_categories
         ), "Invalid categories in excluded categories. Expected format is ['S1', 'S2', ..]"