Cannot read property 'toUpperCase' of null

[cesarin3134]

If I pass and object array and the value of the property is null, i get the this error :

Cannot read property 'toUpperCase' of null

I took a look to the source code and it breaks at the line number 58 :
var letter = scope.items[i][attrs.key].toUpperCase().charAt(0);
this is because the value of scope.items[i][attrs.key] is null.
I think that it would be good if the attrs.key passed is null, It should take the next attribute value of the object.
does somebody know how can i fix it ?
thanks.