diff --git a/kitty/fast_data_types.pyi b/kitty/fast_data_types.pyi
index 954995878..96625808a 100644
--- a/kitty/fast_data_types.pyi
+++ b/kitty/fast_data_types.pyi
@@ -1682,10 +1682,10 @@ def expand_ansi_c_escapes(test: str) -> str: ...
 def update_tab_bar_edge_colors(os_window_id: int) -> bool: ...
 def mask_kitty_signals_process_wide() -> None: ...
 def is_modifier_key(key: int) -> bool: ...
-def base64_encode(src: ReadableBuffer, add_padding: bool = False) -> bytes: ...
-def base64_encode_into(src: ReadableBuffer, output: WriteableBuffer, add_padding: bool = False) -> int: ...
-def base64_decode(src: ReadableBuffer) -> bytes: ...
-def base64_decode_into(src: ReadableBuffer, output: WriteableBuffer) -> int: ...
+def base64_encode(src: Union[str, ReadableBuffer], add_padding: bool = False) -> bytes: ...
+def base64_encode_into(src: Union[str, ReadableBuffer], output: WriteableBuffer, add_padding: bool = False) -> int: ...
+def base64_decode(src: Union[str, ReadableBuffer]) -> bytes: ...
+def base64_decode_into(src: Union[str, ReadableBuffer], output: WriteableBuffer) -> int: ...
 def cocoa_recreate_global_menu() -> None: ...
 def cocoa_clear_global_shortcuts() -> None: ...
 def update_pointer_shape(os_window_id: int) -> None: ...