diff --git a/src/extension-support/extension-manager.js b/src/extension-support/extension-manager.js
index eed64ce9b..4d4e2d283 100644
--- a/src/extension-support/extension-manager.js
+++ b/src/extension-support/extension-manager.js
@@ -7,32 +7,21 @@ const BlockType = require('./block-type');
 // These extensions are currently built into the VM repository but should not be loaded at startup.
 // TODO: move these out into a separate repository?
 // TODO: change extension spec so that library info, including extension ID, can be collected through static methods
-const Scratch3PenBlocks = require('../extensions/scratch3_pen');
-const Scratch3WeDo2Blocks = require('../extensions/scratch3_wedo2');
-const Scratch3MusicBlocks = require('../extensions/scratch3_music');
-const Scratch3MicroBitBlocks = require('../extensions/scratch3_microbit');
-const Scratch3Text2SpeechBlocks = require('../extensions/scratch3_text2speech');
-const Scratch3TranslateBlocks = require('../extensions/scratch3_translate');
-const Scratch3VideoSensingBlocks = require('../extensions/scratch3_video_sensing');
-const Scratch3Speech2TextBlocks = require('../extensions/scratch3_speech2text');
-const Scratch3Ev3Blocks = require('../extensions/scratch3_ev3');
-const Scratch3MakeyMakeyBlocks = require('../extensions/scratch3_makeymakey');
-// todo: only load this extension once we have a compatible way to load its
-// Vernier module dependency.
-// const Scratch3GdxForBlocks = require('../extensions/scratch3_gdx_for');
 
 const builtinExtensions = {
-    pen: Scratch3PenBlocks,
-    wedo2: Scratch3WeDo2Blocks,
-    music: Scratch3MusicBlocks,
-    microbit: Scratch3MicroBitBlocks,
-    text2speech: Scratch3Text2SpeechBlocks,
-    translate: Scratch3TranslateBlocks,
-    videoSensing: Scratch3VideoSensingBlocks,
-    speech2text: Scratch3Speech2TextBlocks,
-    ev3: Scratch3Ev3Blocks,
-    makeymakey: Scratch3MakeyMakeyBlocks
-    // gdxfor: Scratch3GdxForBlocks
+    pen: () => require('../extensions/scratch3_pen'),
+    wedo2: () => require('../extensions/scratch3_wedo2'),
+    music: () => require('../extensions/scratch3_music'),
+    microbit: () => require('../extensions/scratch3_microbit'),
+    text2speech: () => require('../extensions/scratch3_text2speech'),
+    translate: () => require('../extensions/scratch3_translate'),
+    videoSensing: () => require('../extensions/scratch3_video_sensing'),
+    speech2text: () => require('../extensions/scratch3_speech2text'),
+    ev3: () => require('../extensions/scratch3_ev3'),
+    makeymakey: () => require('../extensions/scratch3_makeymakey')
+    // todo: only load this extension once we have a compatible way to load its
+    // Vernier module dependency.
+    // gdxfor: () => require('../extensions/scratch3_gdx_for')
 };
 
 /**
@@ -133,7 +122,7 @@ class ExtensionManager {
                 return Promise.reject(new Error(message));
             }
 
-            const extension = builtinExtensions[extensionURL];
+            const extension = builtinExtensions[extensionURL]();
             const extensionInstance = new extension(this.runtime);
             return this._registerInternalExtension(extensionInstance).then(serviceName => {
                 this._loadedExtensions.set(extensionURL, serviceName);