csslint - broken box model

I've used CSSLint for my stylesheets and I get one warning that I don't understand.

Consider this CSS Code:

div {
    width: 50px;
    height: 50px;
    border: 1px solid;
}

CSSLint says the following: "Broken box model: using height with border." "Broken box model: using width with border."

Why shouldn't I use width / height with border?