switch not setting new value

I can't seem to get the following code to set a new value for the var nClass. It should return "ad"


function cardClass(tmp) {
var na=tmp.split(":");
var pfx=na[0];
var sfx=na[1];
var nClass;
switch(pfx) {
case 2: nClass='a'+sfx; break;
case 3: nClass='b'+sfx; break;
case 4: nClass='c'+sfx; break;
case 5: nClass='d'+sfx; break;
case 6: nClass='e'+sfx; break;
case 7: nClass='f'+sfx; break;
case 8: nClass='g'+sfx; break;
case 9: nClass='h'+sfx; break;
case 10: nClass='i'+sfx; break;
case 11: nClass='j'+sfx; break;
case 12: nClass='k'+sfx; break;
case 13: nClass='l'+sfx; break;
case 14: nClass='m'+sfx; break;
}
return nClass;
}

document.write( cardClass("2:d") );