As with many quirky errors, this one was only happening in IE and only on two computers. Based on where the error was I assumed it had to do with how the XML was being handled. To make a long story short, the problem had to do with my using Microsoft.XMLDOM to create the XML document for jQuery to process. Apparently on these two computers, this version did not properly format the XML. By instead specifying to use Msxml2.DOMDocument.4.0, the error went away.