type ID must be an NCName when namespaces are enabled

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I created a dtd that uses ID & IDREF. When checking to see if the dtd was welformed I got the following error:"type ID must be an NCName when namespaces are enabled"So I did some searches to find examples. I found some and they return the same error.The following dtd and xml are from one of the examples.Why would it be returning this error?<?xml version = "1.0" encoding="UTF-8" standalone = "yes"?><!DOCTYPE CONTACTS [ <!ELEMENT CONTACTS ANY> <!ELEMENT CONTACT (NAME, EMAIL)> <!ELEMENT NAME (#PCDATA)> <!ELEMENT EMAIL (#PCDATA)> <!ATTLIST CONTACT CONTACT_NUM ID #REQUIRED> <!ATTLIST CONTACT MOTHER IDREF #IMPLIED> ]><CONTACTS> <CONTACT CONTACT_NUM="2"> <NAME>Teri Mancuso</NAME> <EMAIL>teri@teri.com</EMAIL> </CONTACT> <CONTACT CONTACT_NUM="1" MOTHER="2"> <NAME>Kristin Mancuso</NAME> <EMAIL>kristin@kristin.com</EMAIL> </CONTACT></CONTACTS>