Are you sure you are passing in a value - paraphrasing your procedure

CREATE PROC dbo.myproc

@Exists INT

AS

IF @EXISTS = 1

BEGIN

PRINT 'UPDATE METHOD HERE'

END

IF @EXISTS =0

BEGIN

PRINT 'INSERT METHOD HERE'

END

dbo.myproc NULL -- imitating passing of a NULL value ... nothing will happen and no error will be reported.