"undefined" when using innerHTML in IE 6

I have a problem where my the attached JS function gives me this in FF 3: "4,593.75", but gives me this in IE 6: "undefined,undefinedundefinedundefined.75"

I have stepped through the function and it is doing it's job properly, but IE won't show the digits properly.  All I'm trying to do is to insert commas in appropriate places for monetary values.

Has anyone run into this before? Is there a workaround for IE 6?

(DECLARATION)
function toMonetaryString(x) {	//put in commas where needed, allow for decimal point
  var _x=x.toString();
  var _len=_x.length 1;
  var dot=_x.indexOf('.');
  var _ret="";
  var _start = 0;
  if( dot >= 0 ) {
	_start = dot-1;
	_ret = _x.substr(dot,_x.length);
  }
  var _cnt = 0;
  for(var i=_start;i>=0;i--) {
	_ret=_x[i] _ret;
	_cnt  ;
	if(!(_cnt%3)) {
	  _ret="," _ret;
	  _cnt = 0;
	}
  }
  return _ret;
			}
... (USAGE)
	  var additionalPeople = parseInt(selectedHouseholdSize) - 12;
	  var amt = (5743.75   (additionalPeople * 975.00));
	  var roundedAmt = Math.round(amt*100)/100;
	  btoaLimitAmt = toMonetaryString(roundedAmt);
	}
	document.getElementById('btoaIncomeLimit').innerHTML = btoaLimitAmt;
... (HTML)
    Is your household MONTHLY gross income at or below $ <span id="btoaIncomeLimit"></span>&nbsp;

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