diff --git a/lib/extension-wrapper.js b/lib/extension-wrapper.js
index 38f1113..546d160 100644
--- a/lib/extension-wrapper.js
+++ b/lib/extension-wrapper.js
@@ -62,7 +62,18 @@ process.on("message", function(message) {
     script = require(`${cwd}/${fileName}`);
   }
 
-  script[message[1][0]](message[1].slice(1), function(err, res) {
+  const funcName = message[1][0];
+
+  if (!script[funcName]) {
+    let fault = {
+      code: "ext",
+      message: `No such function '${funcName}' in extension '${fileName}'`
+    };
+    process.send([message[0], fault]);
+    return;
+  }
+
+  script[funcName](message[1].slice(1), function(err, res) {
     if (!jobs.delete(message[0]))
       return;