NullReferenceException when trying to link 2 scripts

This is probably a newb question, but what I'm trying to do is create a function in one script, in this case 'Designer.js':

 #pragma strict
 // Initialize map. 
 function Initialize (Width:int, Height:int, defChar:String) {
     var W: int = Width;
     var H: int = Height;
     var map = new String[W, H];
     for (var x = 0; x < W; x++) {
         for (var y = 0; y < H; y++) {
             map[x, y] = defChar;
             }
         }
     return map;
     }

Then having another script call it, in this case 'NewlineTest.js':

 #pragma strict
 var mapTester: String[,];
 
 function Start () {
 var designerTestInst: Designer;
 mapTester = designerTestInst.Initialize (10, 40, '#');
 }

Both scripts are tacked onto a player character right now, and whenever I try to test this, it throws 'NullReferenceException: Object reference not set to an instance of an object'.

How do I fix this, and more importantly, what am I doing wrong here?