diff --git a/src/extension-support/tw-default-extension-urls.js b/src/extension-support/tw-default-extension-urls.js
index 7b05eb80..de3c3ec7 100644
--- a/src/extension-support/tw-default-extension-urls.js
+++ b/src/extension-support/tw-default-extension-urls.js
@@ -1,15 +1,18 @@
 // If a project uses an extension but does not specify a URL, it will default to
 // the URLs given here, if it exists. This is useful for compatibility with other mods.
 
-const defaults = new Map();
+const defaults = {
+    // Box2D (`griffpatch`) is not listed here because our extension is not actually
+    // compatible with the original version due to fields vs inputs.
 
-// Box2D (`griffpatch`) is not listed here because our extension is not actually
-// compatible with the original version due to fields vs inputs.
+    // Scratch Lab Animated Text - https://lab.scratch.mit.edu/text/
+    text: 'https://extensions.turbowarp.org/lab/text.js',
 
-// Scratch Lab Animated Text - https://lab.scratch.mit.edu/text/
-defaults.set('text', 'https://extensions.turbowarp.org/lab/text.js');
+    // Turboloader's AudioStream
+    audiostr: 'https://extensions.turbowarp.org/turboloader/audiostream.js',
 
-// Turboloader's AudioStream
-defaults.set('audiostr', 'https://extensions.turbowarp.org/turboloader/audiostream.js');
+    // https://scratch.mit.edu/discuss/topic/842592/
+    faceSensing: 'https://extensions.turbowarp.org/lab/face-sensing.js'
+};
 
 module.exports = defaults;
diff --git a/src/virtual-machine.js b/src/virtual-machine.js
index 4a8f55cf..2cc5d260 100644
--- a/src/virtual-machine.js
+++ b/src/virtual-machine.js
@@ -779,7 +779,10 @@ class VirtualMachine extends EventEmitter {
                 this.extensionManager.loadExtensionIdSync(extensionID);
             } else {
                 // Custom extension
-                const url = extensionURLs.get(extensionID) || defaultExtensionURLs.get(extensionID);
+                let url = extensionURLs.get(extensionID);
+                if (!url && Object.prototype.hasOwnProperty.call(defaultExtensionURLs, extensionID)) {
+                    url = defaultExtensionURLs[extensionID];
+                }
                 if (!url) {
                     throw new Error(`Unknown extension: ${extensionID}`);
                 }