Cannot read property 'chunkFilename' of undefined

[mnpenner]

I'm getting this error when attempting to compile my project for webpack 4 with mini-css-extract-plugin

ERROR in chunk main [entry]
main.e2e70e8fb55738755967.css
Cannot read property 'chunkFilename' of undefined

I don't see any other error details, so I'm not sure where I would even begin to look for the problem.

I've set up mini-css like this:

        plugins: [
            new MiniCssExtractPlugin({
                filename: '[name].[hash].css',
                chunkFilename: 'chunk.[chunkhash].css',
            }),
        ],
        module: {
            rules: [
                {
                    test: /\.less$/,
                    use: [MiniCssExtractPlugin.loader, ...cssLoaders, lessLoader],
                },
                {
                    test: /\.css$/,
                    use: [MiniCssExtractPlugin.loader, ...cssLoaders],
                },
            ],
        },

The loaders look like this if you want to see those too:

let cssLoaders = [
    {
        loader: 'css-loader',
        options: {
            sourceMap: cssSourceMaps,
            root: publicDir,
            localIdentName: '[name]_[local]--[hash:base64:5]',
            importLoaders: 2,
        }
    },
    {
        loader: 'postcss-loader',
        options: {
            sourceMap: cssSourceMaps ? 'inline' : false,
        }
    }
];

let lessLoader = {
    loader: 'less-loader',
    options: {
        sourceMap: cssSourceMaps, // https://github.com/webpack-contrib/less-loader#sourcemaps
        strictMath: true,
        strictUnits: true,
    }
};

I've tried setting the filenames the same as the README too:

      filename: "[name].css",
      chunkFilename: "[id].css"

But I get the same error.

I've got dozens of .less files, so I'm not sure which one, if any, is tripping up the plugin.

[mnpenner]

Going to close this for now. I think it might be chunk-manifest-webpack-plugin actually.