Question from the Algorithmics - Fundamentals test

We have a table and we want to know if the value 42 is there. For this, we have the following code:

    function findNum(array, 42, start, end) 
    {
            If start > end then 
            {
                display error;
            }
            Else 
            {    
                middle = (start+end)/2;
            }
            If array[middle] = 42
            {
               display ''we found 42'';
            }
            If array[middle]>42 
            { 
                findNum(array,42,start,middle-1);
            }
            Else
            {
                 findNum(array,42, middle+1, end);
            }

What must be checked for this binary search to work?