diff --git a/stdlib/2and3/ftplib.pyi b/stdlib/2and3/ftplib.pyi
index 13825c4a8..0e484a0cb 100644
--- a/stdlib/2and3/ftplib.pyi
+++ b/stdlib/2and3/ftplib.pyi
@@ -134,7 +134,7 @@ class FTP:
     def rename(self, fromname: Text, toname: Text) -> str: ...
     def delete(self, filename: Text) -> str: ...
     def cwd(self, dirname: Text) -> str: ...
-    def size(self, filename: Text) -> str: ...
+    def size(self, filename: Text) -> Optional[int]: ...
     def mkd(self, dirname: Text) -> str: ...
     def rmd(self, dirname: Text) -> str: ...
     def pwd(self) -> str: ...