diff --git a/fs_s3fs/_s3fs.py b/fs_s3fs/_s3fs.py
index d7f98e2..a30720f 100644
--- a/fs_s3fs/_s3fs.py
+++ b/fs_s3fs/_s3fs.py
@@ -151,7 +151,7 @@ def readall(self):
         return self._f.readall()
 
     def readinto(self, b):
-        return self._f.readinto()
+        return self._f.readinto(b)
 
     def write(self, b):
         if not self.__mode.writing: