diff --git a/stdlib/string.pyi b/stdlib/string.pyi
index 1a875a071..8b60243f2 100644
--- a/stdlib/string.pyi
+++ b/stdlib/string.pyi
@@ -80,4 +80,4 @@ class Formatter:
     def get_value(self, key: int | str, args: Sequence[Any], kwargs: Mapping[str, Any]) -> Any: ...
     def check_unused_args(self, used_args: set[int | str], args: Sequence[Any], kwargs: Mapping[str, Any]) -> None: ...
     def format_field(self, value: Any, format_spec: str) -> Any: ...
-    def convert_field(self, value: Any, conversion: str) -> Any: ...
+    def convert_field(self, value: Any, conversion: str | None) -> Any: ...