XML Parsing failed with UTF-8

We use dbms_xmlparser to retrieve data from xml files with UTF-8 encoding.  We generally have no issues with special characters. But we recently had a

ORA-31011: XML parsing failed

ORA-19202: Error occurred in XML processing

invalid character 0 (U+0000) found in a Name or Nmtoken

Error at line 231

Usually these are due to easily seen characters that can't be resolved by the parser.

But in this case the offending element content is 'orders@matrixgn.com' without the quotes.

If I remove 'gn' before the '.com' from the content, it works fine. So what is the issue with these 2 chars after the 'x' and before '.com'?