diff --git a/stdlib/3/os/__init__.pyi b/stdlib/3/os/__init__.pyi
index fb4e8b2fa..eee6d1984 100644
--- a/stdlib/3/os/__init__.pyi
+++ b/stdlib/3/os/__init__.pyi
@@ -316,7 +316,7 @@ def putenv(key: Union[bytes, Text], value: Union[bytes, Text]) -> None: ...
 def unsetenv(key: Union[bytes, Text]) -> None: ...
 
 # Return IO or TextIO
-def fdopen(fd: int, mode: str = ..., buffering: int = ..., encoding: str = ...,
+def fdopen(fd: int, mode: str = ..., buffering: int = ..., encoding: Optional[str] = ...,
            errors: str = ..., newline: str = ..., closefd: bool = ...) -> Any: ...
 def close(fd: int) -> None: ...
 def closerange(fd_low: int, fd_high: int) -> None: ...