diff --git a/src/extension-support/extension-manager.js b/src/extension-support/extension-manager.js
index 93cc8957ed9..966ffa8ea19 100644
--- a/src/extension-support/extension-manager.js
+++ b/src/extension-support/extension-manager.js
@@ -277,7 +277,7 @@ class ExtensionManager {
 
     /**
      * Regenerate blockinfo for any loaded extensions
-     * @param {string=} optExtensionId Optional extension ID for refreshing
+     * @param {string} [optExtensionId] Optional extension ID for refreshing
      * @returns {Promise} resolved once all the extensions have been reinitialized
      */
     refreshBlocks (optExtensionId) {
@@ -289,7 +289,10 @@ class ExtensionManager {
             .catch(e => {
                 log.error('Failed to refresh built-in extension primitives', e);
             });
-        if (optExtensionId && this._loadedExtensions.has(optExtensionId)) {
+        if (optExtensionId) {
+            if (!this._loadedExtensions.has(optExtensionId)) {
+                throw new Error(`Unknown extension: ${optExtensionId}`);
+            }
             return refresh(this._loadedExtensions.get(optExtensionId));
         }
         const allPromises = Array.from(this._loadedExtensions.values()).map(refresh);