diff --git a/eelbrain/_data_obj.py b/eelbrain/_data_obj.py
index 3074e59d2..74c7b29be 100644
--- a/eelbrain/_data_obj.py
+++ b/eelbrain/_data_obj.py
@@ -10227,7 +10227,7 @@ def _array_index(self, arg, allow_vertex=True):
                 if allow_vertex:
                     vertex_index = [self._array_index_for_vertex(v) for v in vertices]
                     if label_index is None:
-                        return label_index
+                        return vertex_index
                     else:
                         label_index[vertex_index] = True
                         return label_index