diff --git a/src/highdicom/seg/sop.py b/src/highdicom/seg/sop.py
index 33b5b149..f28218fc 100644
--- a/src/highdicom/seg/sop.py
+++ b/src/highdicom/seg/sop.py
@@ -2174,7 +2174,7 @@ def _get_ref_instance_uids(self) -> List[Tuple[str, str, str]]:
                 f'duplicated referenced SOP Instance UID items: {display_str}.'
             )
 
-        return list(unique_instance_data)
+        return unique_instance_data
 
     def _build_luts(self) -> None:
         """Build lookup tables for efficient querying.