diff --git a/pandas/_typing.py b/pandas/_typing.py
index 172b30c59fc13..ef68018f2721a 100644
--- a/pandas/_typing.py
+++ b/pandas/_typing.py
@@ -314,7 +314,7 @@ def readline(self) -> bytes: ...
 
 
 class WriteExcelBuffer(WriteBuffer[bytes], Protocol):
-    def truncate(self, size: int | None = ...) -> int: ...
+    def truncate(self, size: int | None = ..., /) -> int: ...
 
 
 class ReadCsvBuffer(ReadBuffer[AnyStr_co], Protocol):