Smart step into is disabled

I'm trying to do the Smart Step Into feature of the debugger. But it always seems to be disabled. I have this code below. And I put a breakpoit on the first line. All the other step features work. But Smart Step Into is disabled in the menu. What's wrong?

echo getOutput( calculate( getSomething() ));

function getOutput($value)
{
    return 'Output: ' . $value;
}

function calculate($value)
{
    $value += 5;
}

function getSomething()
{
    return 7;
}