nlapiSearchRecord Not Working on Leadform

Within an external leadform, we are calling the nlapiSearchRecord function to determine if another customer is already set up with the same email address as the prospective new customer.  The same code base works fine on internal forms but does not work in this case.

We are getting back the following message:"Reference Error: NLXMLHttpRequest is not defined."

Here's the code:

try
{
var testemail = nlapiGetFieldValue('email');
var filter = new nlobjSearchFilter( 'email', null, 'is', testemail);
var column = new nlobjSearchColumn( 'email');
var searchresults = nlapiSearchRecord( 'customer', null, filter, column);
if (searchresults.length > 1)
{
  return false;
}
}
catch(err)
{
alert(err.value);
}