diff --git a/data_only_viz/arkit_topology.py b/data_only_viz/arkit_topology.py
index 3fa2dc3..b38a1eb 100644
--- a/data_only_viz/arkit_topology.py
+++ b/data_only_viz/arkit_topology.py
@@ -21,7 +21,7 @@ def encode_topology(names: list[str], parents: list[int]) -> bytes:
     return bytes(out)
 
 
-def decode_topology(payload: bytes):
+def decode_topology(payload: bytes) -> tuple[list[str], list[int]] | None:
     """Return (joint_names, parents) or None if malformed."""
     if len(payload) < 2:
         return None
diff --git a/data_only_viz/tests/test_arkit_topology.py b/data_only_viz/tests/test_arkit_topology.py
index 8f30e19..a025ebb 100644
--- a/data_only_viz/tests/test_arkit_topology.py
+++ b/data_only_viz/tests/test_arkit_topology.py
@@ -26,3 +26,8 @@ def test_rejects_truncated():
 
 def test_rejects_too_short():
     assert decode_topology(b"\x00") is None
+
+
+def test_rejects_truncated_parents():
+    # 1 joint "a", parents section missing.
+    assert decode_topology(b"\x00\x01\x01a") is None