diff --git a/telethon/network/mtproto_sender.py b/telethon/network/mtproto_sender.py
index 400ea6cf..6d103c7a 100644
--- a/telethon/network/mtproto_sender.py
+++ b/telethon/network/mtproto_sender.py
@@ -158,10 +158,11 @@ class MtProtoSender:
             msg_key = reader.read(16)
 
             key, iv = utils.calc_key(self.session.auth_key.key, msg_key, False)
-            if reader.read(len(body) - reader.tell_position()) % 16 != 0:
+            data = reader.read(len(body) - reader.tell_position())
+            if len(data) % 16 != 0:
                 raise SecurityError('AES block size missmatch')
             plain_text = AES.decrypt_ige(
-                reader.read(len(body) - reader.tell_position()), key, iv)
+                data, key, iv)
 
             with BinaryReader(plain_text) as plain_text_reader:
                 plain_text_reader.read_long()  # remote_salt