Webpack CommonsChunkPlugin issue

a.js

import c from "./c";

export default "a" + c;

b.js

import c from "./c";

export default "b" + c;

c.js

export default "c";

d.js

export default "d";

main.js

//A and B are largely the same code (shared dependency on C)
require.ensure([], (require) => {
    const a = require("./a").default;
    console.log("Got a", a)
}, "aChunk");

require.ensure([], (require) => {
    const b = require("./b");
    console.log("Got b", b)
}, "bChunk");

// D is unrelated to A or B (no shared code), but prevents any code from being extracted from A or B
require.ensure([], (require) => {
    const d = require("./d");
    console.log("Got d", d);
}, "dChunk");

webpack.config.js

const path = require("path");
const webpack = require("webpack");

module.exports = {
    entry: "./main",
    output: {
        filename: "bundle.js",
        chunkFilename: "[name].bundle.js",
        path: path.resolve("dist")
    },
    plugins: [
        new webpack.optimize.CommonsChunkPlugin({
            name: "main",
            async: "common"
        })
    ]
}

Running this as is, I expect to see common code from aChunk and bChunk to have been extracted, but it hasn't:

           Asset       Size  Chunks             Chunk Names
bChunk.bundle.js  601 bytes       0  [emitted]  bChunk
aChunk.bundle.js  601 bytes       1  [emitted]  aChunk
dChunk.bundle.js  271 bytes       2  [emitted]  dChunk
       bundle.js    6.51 kB       3  [emitted]  main

If I comment out lines 13-16 (where d is required), however, I get the expected behavior:

common.bundle.js  190 bytes       0  [emitted]  common
bChunk.bundle.js  411 bytes       1  [emitted]  bChunk
aChunk.bundle.js  417 bytes       2  [emitted]  aChunk
       bundle.js    6.51 kB       3  [emitted]  main