diff --git a/myst_nb/execution.py b/myst_nb/execution.py
index ad66fdfc..53db4c6c 100644
--- a/myst_nb/execution.py
+++ b/myst_nb/execution.py
@@ -14,7 +14,7 @@
 import tempfile
 from datetime import datetime
 from pathlib import Path
-from typing import List, Optional, Set
+from typing import List, Optional, Set, Tuple
 
 import nbformat as nbf
 from jupyter_cache import get_cache
@@ -327,7 +327,9 @@ def _converter(path):
     return result
 
 
-def nb_has_all_output(source_path: str, nb_extensions: List[str] = (".ipynb",)) -> bool:
+def nb_has_all_output(
+    source_path: str, nb_extensions: Tuple[str] = (".ipynb",)
+) -> bool:
     """Determine if the path contains a notebook with at least one output."""
     has_outputs = False
     ext = os.path.splitext(source_path)[1]