In SASS, I can't @extend from a class with a number in it

[krivaten]

When trying to run csscomb on a folder of scss files, I've noticed that I get an error whenever I @extend from a class with a number in it. For example:

.footer-nav {
    > li {
        @extend .col-md-4;
    }
}

Everywhere else I use @extend works fine, and and so does this when I remove the number at the end of .col-md-4. Did I perhaps overlook something in my settings, or is this an issue with the plugin?