diff --git a/src/openai/lib/streaming/responses/_responses.py b/src/openai/lib/streaming/responses/_responses.py
index 6975a9260d..513ca5dbb5 100644
--- a/src/openai/lib/streaming/responses/_responses.py
+++ b/src/openai/lib/streaming/responses/_responses.py
@@ -328,18 +328,20 @@ def accumulate_event(self, event: RawResponseStreamEvent) -> ParsedResponseSnaps
             return self._create_initial_response(event)
 
         if event.type == "response.output_item.added":
-            if event.item.type == "function_call":
+            item = event.item
+            if item is None:
+                return snapshot
+
+            if item.type == "function_call":
                 snapshot.output.append(
-                    construct_type_unchecked(
-                        type_=cast(Any, ParsedResponseFunctionToolCall), value=event.item.to_dict()
-                    )
+                    construct_type_unchecked(type_=cast(Any, ParsedResponseFunctionToolCall), value=item.to_dict())
                 )
-            elif event.item.type == "message":
+            elif item.type == "message":
                 snapshot.output.append(
-                    construct_type_unchecked(type_=cast(Any, ParsedResponseOutputMessage), value=event.item.to_dict())
+                    construct_type_unchecked(type_=cast(Any, ParsedResponseOutputMessage), value=item.to_dict())
                 )
             else:
-                snapshot.output.append(event.item)
+                snapshot.output.append(item)
         elif event.type == "response.content_part.added":
             output = snapshot.output[event.output_index]
             if output.type == "message":
diff --git a/tests/lib/responses/test_responses.py b/tests/lib/responses/test_responses.py
index 8e5f16df95..8f5923c580 100644
--- a/tests/lib/responses/test_responses.py
+++ b/tests/lib/responses/test_responses.py
@@ -8,6 +8,10 @@
 
 from openai import OpenAI, AsyncOpenAI
 from openai._utils import assert_signatures_in_sync
+from openai._types import omit
+from openai._models import construct_type_unchecked
+from openai.types.responses import ResponseStreamEvent
+from openai.lib.streaming.responses._responses import ResponseStreamState
 
 from ...conftest import base_url
 from ..snapshots import make_snapshot_request
@@ -61,3 +65,40 @@ def test_parse_method_definition_in_sync(sync: bool, client: OpenAI, async_clien
         checking_client.responses.parse,
         exclude_params={"tools"},
     )
+
+
+def test_stream_accumulator_ignores_missing_output_item() -> None:
+    state = ResponseStreamState(input_tools=omit, text_format=omit)
+
+    state.handle_event(
+        construct_type_unchecked(
+            type_=ResponseStreamEvent,
+            value={
+                "type": "response.created",
+                "sequence_number": 0,
+                "response": {
+                    "id": "resp_123",
+                    "object": "response",
+                    "created_at": 0,
+                    "status": "in_progress",
+                    "model": "gpt-4o-mini",
+                    "output": [],
+                    "parallel_tool_calls": True,
+                    "tool_choice": "auto",
+                    "tools": [],
+                },
+            },
+        )
+    )
+
+    event = construct_type_unchecked(
+        type_=ResponseStreamEvent,
+        value={
+            "type": "response.output_item.added",
+            "sequence_number": 1,
+            "output_index": 0,
+            "item": None,
+        },
+    )
+
+    assert state.handle_event(event) == [event]