TextFields Based On User Selection - Variables And Objects

I have 3 TextFields, called txtUSD, txtEUR, txtAUS. And a PopupList with the same values, minus the txt part, but I need to form the names of the TextFields to use based on the selection that the user made. So I've done this:

Code:
function btConvert_Click(event){
var amount = document.getElementById("txtAmount").value;
var rates = document.getElementById("lstConvertTo").value;
var from = "txt" + document.getElementById("lstFrom").options[document.getElementById('lstFrom').selectedIndex].text;
var to = "txt" + document.getElementById("lstConvertTo").options[document.getElementById("lstConvertTo").selectedIndex].text;
var curr_from = document.getElementById(from).value;
var curr_to = document.getElementById(to).value;
if(curr_from > curr_to){
amount * rates;
} else {
amount / rates;
} alert(result);
}

But every time I try it I get this error:
Code:
mobile/main.js line 215: Result of expression 'document.getElementById(from)' [null] is not an object.