diff --git a/src/static/js/skiplist.js b/src/static/js/skiplist.js
index 798991ad9..105f5902c 100644
--- a/src/static/js/skiplist.js
+++ b/src/static/js/skiplist.js
@@ -95,6 +95,11 @@ class Point {
   }
 
   insert(entry) {
+    if (entry.key == null) throw new Error('entry.key must not be null');
+    if (this._skipList.containsKey(entry.key)) {
+      throw new Error(`an entry with key ${entry.key} already exists`);
+    }
+
     const newNode = new Node(entry);
     const pNodes = this.nodes;
     const pIdxs = this.idxs;
diff --git a/src/tests/frontend/index.html b/src/tests/frontend/index.html
index 3b0f1c6e0..6e2f8ca77 100644
--- a/src/tests/frontend/index.html
+++ b/src/tests/frontend/index.html
@@ -9,6 +9,7 @@
     <div id="mocha"></div>
     <div id="iframe-container"></div>
 
+    <script src="/static/js/require-kernel.js"></script>
     <script src="/static/js/vendors/jquery.js"></script>
     <script src="/static/js/vendors/browser.js"></script>
     <script src="/static/plugins/js-cookie/src/js.cookie.js"></script>
diff --git a/src/tests/frontend/runner.js b/src/tests/frontend/runner.js
index d9f37bcae..b119f2963 100644
--- a/src/tests/frontend/runner.js
+++ b/src/tests/frontend/runner.js
@@ -159,6 +159,11 @@ $(() => {
   // get the list of specs and filter it if requested
   const specs = specs_list.slice();
 
+  const absUrl = (url) => new URL(url, window.location.href).href;
+  require.setRootURI(absUrl('../../javascripts/src'));
+  require.setLibraryURI(absUrl('../../javascripts/lib'));
+  require.setGlobalKeyPath('require');
+
   // inject spec scripts into the dom
   const $body = $('body');
   $.each(specs, (i, spec) => {
diff --git a/src/tests/frontend/specs/skiplist.js b/src/tests/frontend/specs/skiplist.js
new file mode 100644
index 000000000..4b2ebd5aa
--- /dev/null
+++ b/src/tests/frontend/specs/skiplist.js
@@ -0,0 +1,18 @@
+'use strict';
+
+const SkipList = require('ep_etherpad-lite/static/js/skiplist');
+
+describe('skiplist.js', function () {
+  it('rejects null keys', async function () {
+    const skiplist = new SkipList();
+    for (const key of [undefined, null]) {
+      expect(() => skiplist.push({key})).to.throwError();
+    }
+  });
+
+  it('rejects duplicate keys', async function () {
+    const skiplist = new SkipList();
+    skiplist.push({key: 'foo'});
+    expect(() => skiplist.push({key: 'foo'})).to.throwError();
+  });
+});