"TypeError: oForm is undefined" in Firefox/Safari/Chrome

"TypeError: oForm is undefined" in Firefox/Safari/Chrome

The following code only works in IE with no errors but throws a "TypeError: oForm is undefined" in Firefox and doesn't appear to work in Safari or Chrome either.
 
Currently using jQuery and jQueryUI 1.11.1.min.js
 
Snippet (on anything but IE it fails at line 4)
  1. function submitform(str1,str2,str3) {
  2.       var formname = str1;
  3.       var oForm = document.forms[formname]; 
  4.       var elLength = oForm.elements.length;
The form is:
  1. <form name="memberphoneadd" id="memberphoneadd">
  2. bunch of fields
  3. <a href="javascript:void(0);" class="btn checkmark inf" onclick="FormPost('memberphoneadd','memberphones','member_phones_add','');"><span></span>Save new phone</a>
  4. </form>
Fields:
      str1 = the form's name
      str2 = the div to process the form in
      str3 = the page to submit the form to
 
Any insights would be appreciated.