diff --git a/python/chemiscope/structures/_stk.py b/python/chemiscope/structures/_stk.py
index 4ffb34b60..9f2aab13b 100644
--- a/python/chemiscope/structures/_stk.py
+++ b/python/chemiscope/structures/_stk.py
@@ -1,3 +1,4 @@
+import typing
 from collections import Counter
 
 try:
@@ -9,7 +10,7 @@
 
 
 def _stk_valid_structures(
-    frames: stk.Molecule | list[stk.Molecule],
+    frames: typing.Union(stk.Molecule, list[stk.Molecule]),
 ) -> tuple[list[stk.Molecule], bool]:
     if HAVE_STK and isinstance(frames, stk.Molecule):
         # deal with the user passing a single frame