diff --git a/google/cloud/bigtable/data/_cross_sync/_decorators.py b/google/cloud/bigtable/data/_cross_sync/_decorators.py
index e87b6339d..f37b05b64 100644
--- a/google/cloud/bigtable/data/_cross_sync/_decorators.py
+++ b/google/cloud/bigtable/data/_cross_sync/_decorators.py
@@ -308,7 +308,7 @@ def __init__(
         sync_name: str | None = None,
         *,
         replace_symbols: dict[str, str] | None = None,
-        docstring_format_vars: dict[str, tuple[str, str]] | None = None,
+        docstring_format_vars: dict[str, tuple[str | None, str | None]] | None = None,
         rm_aio: bool = True,
     ):
         super().__init__(