PowerQuery Extension API: Web.Contents parsing binary as JSON

[Moof]

I'm using the PowerQuery Extension API (NOT Power BI Desktop) and I have the following function:

shared PQDeiworld.Contents = (url as text, DEINSP as text) =>
    let
		DEIPass = Extension.CurrentCredential()[Key],
        query = [
			DEINSP = DEINSP,
			FlagIdiomas = 1,
			DEIPass = DEIPass
		], 
		answer = Text.FromBinary(Web.Contents(url, [Query = query]))
    in
        answer;

The response to this from the (internal) web server is a slightly malformed XML Document, that I know how to clean up in PBI Dekstop.

When I try to run the extension in Visual Studio, I get the following response:

Unexpected Exception

Details:

[DataFormat.Error] We found extra characters at the end of the JSON Input

I'm not telling Web.Contents to parse JSON, I want to get the binary out, and parse it myself - how do I stop Web.Contents from doing so?