--- a/testing/marionette/evaluate.js
+++ b/testing/marionette/evaluate.js
@@ -109,17 +109,17 @@ evaluate.sandbox = function(sb, script, 
       sandboxName = null,
       timeout = DEFAULT_TIMEOUT,
     } = {}) {
   let scriptTimeoutID, timeoutHandler, unloadHandler;
 
   let promise = new Promise((resolve, reject) => {
     let src = "";
     sb[COMPLETE] = resolve;
-    timeoutHandler = () => reject(new ScriptTimeoutError("Timed out"));
+    timeoutHandler = () => reject(new ScriptTimeoutError(`Timed out after ${timeout} ms`));
     unloadHandler = sandbox.cloneInto(
         () => reject(new JavaScriptError("Document was unloaded")),
         sb);
 
     // wrap in function
     if (!directInject) {
       if (async) {
         sb[CALLBACK] = sb[COMPLETE];