What am I doing wrong here (v10.2)?

//Global Variable
var webtab;

function SwitchTab() {
var firstTab = 0;
webtab.setSelectedTab(firstTab);
}

I keep on getting the following error "Object doesn't support this property or method" when it hits the selected tab section. I have tried explictly stating 0 in parenthesis and also using "webtab[0]" in parenthesis.