XMLException with Google AdWords API

So I am trying to use the mutate method of ad group service and cannot resolve an issue I am having with it. Whenever the line

    retVal = adGroupService.mutate(operations.ToArray());

[where

    operations = new List<AdGroupOperation>();

]

is called, I get the following error message

    An exception of type 'System.Xml.XmlException' occurred in 
    System.Xml.dll but was not handled in user code

    Additional information: Data at the root level is invalid. Line 1, position 1.

Immediately before the above line, I have placed the code

    XmlDocument doc = new XmlDocument();
    doc.Load(@"file-path");

to test whether there if a problem with my web.config, which is where the application is getting the relevant AdWords settings, eg the Developer Token. This loads no problem, and I can breakpoint and check 'doc', and all is in order.

Does anyone have any suggestions? I am working in C# if that is at all helpful. Please let me know if further details are needed.

Thanks, Conor