CSS minifier error: CSSO & Webpack

Question

I'm getting the following error as I bundle my js and css with Webpack.

ERROR in Cannot find module './prepare/AppActions.js'
 @ ./components/filename.css 4:14-84

This error occurs no matter which css file I import, which makes me think it's caused by csso (https://github.com/css/csso), rather than the css files themselves. Also, I did a text search on all files in the project, and found prepare/AppActions.js in csso/css-browser.js.

Grid_component.js that imports the css file

var LinkedStateMixin = require('react-addons-linked-state-mixin');
import {AgGridReact} from 'ag-grid-react-component';
import moment from 'moment';
var myConst = require('../actions/myConst');
import Purger from '../components/Purger';
import './filename.css';
import './theme-fresh.css';

Here is a little more info on my dev environment.

npm -v 3.8.0
webpack 1.13
: css loader defined as { test: /.css$/, loader: "style-loader!css-loader" }
node -v v5.3.0