diff --git a/packages/webamp/js/setImmediate.ts b/packages/webamp/js/setImmediate.ts
new file mode 100644
index 00000000..26f5db4c
--- /dev/null
+++ b/packages/webamp/js/setImmediate.ts
@@ -0,0 +1,127 @@
+// A Fork of the NPM module `setimmediate`
+// I've adapted this to only include the browser implementation and to not
+// mutate the global object and instead export the `setImmediate` function.
+
+type Handle = { callback: Function; args: any[] };
+type TaskByHandle = { [handle: number]: Handle };
+
+let nextHandle = 1; // Spec says greater than zero
+const tasksByHandle: TaskByHandle = {};
+let currentlyRunningATask = false;
+let registerImmediate: (arg0: number) => void;
+
+export function setImmediate(callback: Function) {
+  // Copy function arguments
+  const args = new Array(arguments.length - 1);
+  for (let i = 0; i < args.length; i++) {
+    args[i] = arguments[i + 1];
+  }
+  // Store and register the task
+  const task = { callback: callback, args: args };
+  tasksByHandle[nextHandle] = task;
+  registerImmediate(nextHandle);
+  return nextHandle++;
+}
+
+function clearImmediate(handle: number) {
+  delete tasksByHandle[handle];
+}
+
+function run(task: Handle) {
+  const callback = task.callback;
+  const args = task.args;
+  switch (args.length) {
+    case 0:
+      callback();
+      break;
+    case 1:
+      callback(args[0]);
+      break;
+    case 2:
+      callback(args[0], args[1]);
+      break;
+    case 3:
+      callback(args[0], args[1], args[2]);
+      break;
+    default:
+      // eslint-disable-next-line prefer-spread
+      callback.apply(undefined, args);
+      break;
+  }
+}
+
+function runIfPresent(handle: number) {
+  // From the spec: "Wait until any invocations of this algorithm started before this one have completed."
+  // So if we're currently running a task, we'll need to delay this invocation.
+  if (currentlyRunningATask) {
+    // Delay by doing a setTimeout. setImmediate was tried instead, but in Firefox 7 it generated a
+    // "too much recursion" error.
+    setTimeout(runIfPresent, 0, handle);
+  } else {
+    const task = tasksByHandle[handle];
+    if (task) {
+      currentlyRunningATask = true;
+      try {
+        run(task);
+      } finally {
+        clearImmediate(handle);
+        currentlyRunningATask = false;
+      }
+    }
+  }
+}
+
+function canUsePostMessage() {
+  // The test against `importScripts` prevents this implementation from being installed inside a web worker,
+  // where `global.postMessage` means something completely different and can't be used for this purpose.
+  if (!window.importScripts) {
+    let postMessageIsAsynchronous = true;
+    const oldOnMessage = window.onmessage;
+    window.onmessage = function () {
+      postMessageIsAsynchronous = false;
+    };
+    window.postMessage("", "*");
+    window.onmessage = oldOnMessage;
+    return postMessageIsAsynchronous;
+  }
+}
+
+function installPostMessageImplementation() {
+  // Installs an event handler on `global` for the `message` event: see
+  // * https://developer.mozilla.org/en/DOM/window.postMessage
+  // * http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#crossDocumentMessages
+
+  const messagePrefix = `setImmediate$${Math.random()}$`;
+
+  window.addEventListener(
+    "message",
+    (event) => {
+      if (
+        event.source === window &&
+        typeof event.data === "string" &&
+        event.data.indexOf(messagePrefix) === 0
+      ) {
+        runIfPresent(+event.data.slice(messagePrefix.length));
+      }
+    },
+    false
+  );
+
+  registerImmediate = function (handle) {
+    window.postMessage(messagePrefix + handle, "*");
+  };
+}
+
+function installSetTimeoutImplementation() {
+  registerImmediate = function (handle) {
+    setTimeout(runIfPresent, 0, handle);
+  };
+}
+
+if (canUsePostMessage()) {
+  // For non-IE10 modern browsers
+  installPostMessageImplementation();
+} else {
+  // For older browsers
+  installSetTimeoutImplementation();
+}
diff --git a/packages/webamp/js/webamp.ts b/packages/webamp/js/webamp.ts
index b880d1f6..351a6ff6 100644
--- a/packages/webamp/js/webamp.ts
+++ b/packages/webamp/js/webamp.ts
@@ -3,6 +3,24 @@ import * as musicMetadataBrowser from "music-metadata-browser";
 import { Options } from "./types";
 import WebampLazy, { PrivateOptions } from "./webampLazy";
 
+// There is some bug between how JSZip pulls in setimmediate (which it expects
+// to polyfill `window.setimmediate` and our bundler set. The result is that one
+// of our bundles is missing the polyfill. If we call JSZip code from within
+// that bundle the polyfill is not present and we get an error.
+//
+// To work around this we manually import the helper function which is the one
+// place JSZip actually uses `setImmediate` and then set it to use our fork of
+// the polyfill which does not require adding a property to `window`.
+import { setImmediate } from "./setImmediate";
+// @ts-ignore
+import Utils from "jszip/lib/utils";
+// @ts-ignore
+Utils.delay = function (callback, args, self) {
+  setImmediate(() => {
+    callback.apply(self || null, args || []);
+  });
+};
+
 export type * from "./types";
 
 export default class Webamp extends WebampLazy {