Trigger could not find the Control id of a button inside tabcontainer

Hi I have an updatepanel and a tab container inside it. Tab panel consists of two drop downs and a button in first drop down list of universities is coming from DB, and in second drop down list of course comes on postback of first drop down and when I try to save the selected index/value of drop down with course on button click then it always save only 0th index value. I tried to add postback trigger on updatepanel for my button but this error occurs "A control with ID 'btnSaveCourses' could not be found for the trigger in UpdatePanel 'updatePanel1' can anybody help me please???