diff --git a/src/extension-support/extension-manager.js b/src/extension-support/extension-manager.js
index 30bc6d50d..d3ced5df1 100644
--- a/src/extension-support/extension-manager.js
+++ b/src/extension-support/extension-manager.js
@@ -390,6 +390,10 @@ class ExtensionManager {
             }
             break;
         default:
+            if (!blockInfo.opcode) {
+                throw new Error('Missing opcode for block');
+            }
+
             blockInfo.func = blockInfo.func ? this._sanitizeID(blockInfo.func) : blockInfo.opcode;
 
             // Avoid promise overhead if possible