Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler); wont fire the first time page load

hi there
I am using Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler)
well my first time  page load wont fire  endRequest
when i load page second and other times everythink works fine
any idea somebody why first load wont fire endRequest?
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