diff --git a/stdlib/2/_io.pyi b/stdlib/2/_io.pyi
index d0185ae7d..e8e448f2c 100644
--- a/stdlib/2/_io.pyi
+++ b/stdlib/2/_io.pyi
@@ -16,7 +16,7 @@ _T = TypeVar("_T")
 class _IOBase(BinaryIO):
     @property
     def closed(self) -> bool: ...
-    def _checkClosed(self) -> None: ...
+    def _checkClosed(self, msg: Optional[str] = ...) -> None: ...  # undocumented
     def _checkReadable(self) -> None: ...
     def _checkSeekable(self) -> None: ...
     def _checkWritable(self) -> None: ...
diff --git a/stdlib/3/io.pyi b/stdlib/3/io.pyi
index acab18ca1..22a8784a7 100644
--- a/stdlib/3/io.pyi
+++ b/stdlib/3/io.pyi
@@ -45,6 +45,7 @@ class IOBase:
     def __del__(self) -> None: ...
     @property
     def closed(self) -> bool: ...
+    def _checkClosed(self, msg: Optional[str] = ...) -> None: ...  # undocumented
 
 class RawIOBase(IOBase):
     def readall(self) -> bytes: ...