help newbie,maya poly modelling trouble

first,hello all
this is my first post,and i must say i m quite a newbie with cg modelling
here s the problem

i use maya 4.0,and every time i use booleans with and extruded object,for example an extruded cube with a sphere,both objects disappear
how can i fix it,or,how can i attach two objects together?(i tried to use combine,but it wasn t clean enough to be satisfying) thx for your answers