JS Error -> Split is not a function

I'm teaching myself javascript, as we have decided to develop our own analytics tracking to integrate into our analytics db.  We had previously been using a 3rd party to provide tracking technology.  So, this is my first dive into javascript, with my experience mostly in VB .net/asp and SQL.  I was making good progress, and then ran into this issue.  Javascript tested through: var broken_info = cookieValue.split(":")

When the browser executes this line I get: cookieValue.split is not a function in Firefox error console.

Here is the complete javascript (which is housed in a js file) and is called from the javascript at this URL:  http://www.searchmarketingtools.com/tester.html?source=testengine&kw=test+keyword

//<!-- Tracking Code v1.00 - All rights reserved -->

function Randomstring()
{
var chararr="abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"

var rndstr=""
for(i=0;i<10;i++)
{

      rndstr=rndstr+chararr.charAt(Math.round(Math.random()*62)-1)
}
    id = rndstr;
    return rndstr;
}
function Setcookie()
{
    var _d=new Date();
    _d.setTime(_d.getTime()+31536000000);
      expires=_d
    var fcnv = "SMTstatbuilder="+"id:"+Randomstring()+"; path=/; expires="+expires;
    document.cookie = fcnv;


}
function readCookie(name) {
      if (document.cookie == '') {
   // there's no cookie, so go no further
      return false;
      } else {
   // there is a cookie
      var firstChar, lastChar;
      var theBigCookie = document.cookie;
      firstChar = theBigCookie.indexOf(name);
   // find the start of 'name'
      if(firstChar != -1)  {
   // found cookie
      firstChar += name.length + 1;
   // skip 'name' and '='
      lastChar = theBigCookie.indexOf(';', firstChar);
   // Find the end of the value string (i.e. the next ';').
      if(lastChar == -1) lastChar = theBigCookie.length;
      return unescape(theBigCookie.substring(firstChar, lastChar));
            } else {
   // If there was no cookie of that name, return false.
      return false;
      }
    }
}
var id
var cookieValue = readCookie('SMTstatbuilder');
if(cookieValue == "false"){Setcookie}
else {
     var broken_info = cookieValue.split(":")
    id = broken_info[1];
}
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From all the reference/documentation I have reviewed, split is a valid string function.  Why am I getting this error?