Why am I getting an "Unrecognised input..." error when I nest classes?

I'm trying to use nested styles on different classes in a LESS stylesheet, but every time I try to compile, I get ParseError: Unrecognised input in style.less.

I've searched around, and it seems like what I'm doing should be valid. The strange part is that it was working for me yesterday. I've had lots of weird errors in Node applications lately, but I uninstalled and reinstalled Node, NPM, and all of my packages this morning, and that didn't help anything.

.grid {
    overflow:hidden;
    margin:0;
    padding:3em 0 0 0;
    width:100%;
    list-style:none;
    text-align:center;
    font-family:'Helvetica Neue';

    .tile {  // The error occurs on this line
        position:relative;
        z-index:1;
        display:inline-block;
        overflow:hidden;
        margin:-0.135em;

        ...
    }

    ...
}

Here's a link to a CodePen with all of my code. CodePen shows the error, too, so it's not something wrong with my installation.