[VE][338] Error Message Feedback

Validating http://www.librosbudistas.com/temas/index.asp

I get this error
Warning  Line 225, column 55: cannot generate system identifier for general 
entity "vn_url" .

....ref="../cesta/cesta.asp?UC_AddId=CUE&vn_url=../temas/index.asp?tema=nino"><im

An entity reference was found in the document, but there is no reference by 
that name defined. Often this is caused by misspelling the reference name, 
unencoded ampersands, or by leaving off the trailing semicolon (;). The 
most common cause of this error is unencoded ampersands in URLs as 
described by the WDG in "Ampersands in URLs".

Entity references start with an ampersand (&) and end with a semicolon (;). 
If you want to use a literal ampersand in your document you must encode it 
as "&amp;" (even inside URLs!). Be careful to end entity references with a 
semicolon or your entity reference may get interpreted in connection with 
the following text. Also keep in mind that named entity references are 
case-sensitive; &Aelig; and &aelig; are different characters.

If this error appears in some markup generated by PHP's session handling 
code, this article has explanations and solutions to your problem.

Note that in most documents, errors related to entity references will 
trigger up to 5 separate messages from the Validator. Usually these will 
all disappear when the original problem is fixed.

This is a validation mistake because the ampersand is part of a ULR for an 
asp page with more than one variable. The code was...

<a 
href="../cesta/cesta.asp?UC_AddId=CUE&vn_url=../temas/index.asp?tema=nino"><img 
src="../graficos/cesta.gif" alt="poner La Cueva en la cesta de compras" 
width="40" height="39" border="0" align="centre"></a>