diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index 7afd665..0817da9 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1059,7 +1059,7 @@
         # sslobj.read() sometimes returns < size bytes
         data = self.sslobj.read(size)
         while len(data) < size:
-                data += self.sslobj.read(len(data)-size)
+                data += self.sslobj.read(size-len(data))
 
         return data