Some code paths in 'x' return value, some not

Sample code:

//...
function calculate_area($shape, array $sides) { // Some code paths in 'calculate_area' return value, some not
    switch ($shape) {
    case 'square':
        return $sides[0] * $sides[0];
    case 'rectangle':
        return $sides[0] * $sides[1];
    }
}
//...

Not all code paths within function return value. In this example if $shape is for example 'circle' there is no defined return value.