diff --git a/panda/src/pgraph/lightAttrib.I b/panda/src/pgraph/lightAttrib.I
index 0c6686181b..bc02883c7e 100644
--- a/panda/src/pgraph/lightAttrib.I
+++ b/panda/src/pgraph/lightAttrib.I
@@ -42,6 +42,7 @@ get_num_non_ambient_lights() const {
  */
 INLINE NodePath LightAttrib::
 get_on_light(size_t n) const {
+  check_sorted();
   nassertr(n < _sorted_on_lights.size(), NodePath::fail());
   return _sorted_on_lights[n];
 }