'&' is a special character in XML. It's used to represent "entities" such as < (for the '<' character. because that's also a special character). So it's seeing '&zs' and thinks you were intending it to be an entity like '&zs;' - entities always end in a semi-colon.
If you just want a plain ampersand character '&', you need to escape it. To do that, replace it with '&'