diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 993f94bc055b2e2..d352f0f9e0f0a6b 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -1928,7 +1928,7 @@ def truncate(self, pos=None):
         """Truncate size to pos, where pos is an int."""
         self._unsupported("truncate")
 
-    def readline(self):
+    def readline(self, size=-1):
         """Read until newline or EOF.
 
         Returns an empty string if EOF is hit immediately.