diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index 589a025..acdaa55 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -178,7 +178,7 @@ def _content_to_message_param(
 
     return ChatCompletionAssistantMessage(
         role=role,
-        content=_get_content(content.parts),
+        content=_get_content(content.parts) or None,
         tool_calls=tool_calls or None,
     )
 
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 6b09b1c..9d9bd71 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -536,7 +536,7 @@ def test_content_to_message_param_function_call():
   content.parts[0].function_call.id = "test_tool_call_id"
   message = _content_to_message_param(content)
   assert message["role"] == "assistant"
-  assert message["content"] == []
+  assert message["content"] == None
   assert message["tool_calls"][0].type == "function"
   assert message["tool_calls"][0].id == "test_tool_call_id"
   assert message["tool_calls"][0].function.name == "test_function"