ltm send string for monitor

Hi i am running f5 ltm on code 11.5.x and configured below health string for a pool but it did not bring pool up:- GET /xyz.aspx HTTP1.1\r\nHost:server01\r\nConnection: Close\r\n\r\n\r\n

Later on just changing the send string syntax as below; brought the pool up GET /xyz.aspx HTTP/1.1\r\nHost: server01\r\n\r\n

What it did not work with original send string and why it worked with string without "Close..."