Bug when interpreting attribute values

I have chosen not to subscribe to this list. I just want to convey to 
you an issue regarding the W3C HTML Validator:

It may be called a bug, but I am not sure. When validating the fragment
<img src=/grafik/layout/read_more.gif alt="Read more">

the Validator reports an error: "an attribute value must be quoted if 
it contains any character other than letters (A-Za-z), digits, 
hyphens, and periods; use quotes if in doubt"

The underscore is an issue here. But the HTML spec 
http://www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.2 clearly 
states that underscores are allowed without placing quotation marks. 
Surely, I will place quotation marks around this URI, but I believe 
there is a conflict here.

Gunnar Vestergaard