value of attribute "ID" invalid: "_" cannot start a name

I'm finding a potential problem with the HTML Validator.
 
All my ASP.NET (.aspx) pages fail to validate with the following error:
 
            value of attribute "ID" invalid: "_" cannot start a name
 
However, I have no control over this element on the page because it is
dynamically generated by ASP.NET.  Surely MS wouldn't generate HTML
pages that fail to validate.??
 
Interesting note: Even though these pages don't validate, they are
crawled and searchable on Google.
 
Phil