Hi there, I have the following xml file <

<bookshop> <book bid="1"> Programming in C# </book> <book bid="2"> programming in Java </book> <authors>  <author bidref="1"> person1 </author>  <author bidref="2"> person2 </author>  <author bidref="1"> person3 </author> </bookshop>
then I made the following Relax NG schema

start=element bookshop{ element book {attribute bid{xsd:ID},               text} element authors{     element author { attribute bidref{xsd:IDREF},                          text}                }}
However, it always gives me error said that value of attribute bid is invalid must be an XML name without colons