diff --git a/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js b/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
index 40330b99beb..a5d96702e00 100644
--- a/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
+++ b/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
@@ -131,6 +131,9 @@ export function makeXsSubprocessFactory({
     async function issueTagged(item) {
       parentLog(item[0], '...', item.length - 1);
       const result = await worker.issueStringCommand(JSON.stringify(item));
+      // note: if 'result.reply' is empty, that probably means the
+      // worker reached end-of-crank (idle) without seeing the
+      // `dispatch()` promise resolve
       const reply = JSON.parse(result.reply);
       assert(Array.isArray(reply));
       const [tag, ...rest] = reply;
diff --git a/packages/SwingSet/src/liveslots/liveslots.js b/packages/SwingSet/src/liveslots/liveslots.js
index c60813bc81f..7a3e624a43d 100644
--- a/packages/SwingSet/src/liveslots/liveslots.js
+++ b/packages/SwingSet/src/liveslots/liveslots.js
@@ -1275,7 +1275,7 @@ function build(
     );
 
     // here we finally invoke the vat code, and get back the root object
-    const rootObject = buildRootObject(
+    const rootObject = await buildRootObject(
       harden(vpow),
       harden(vatParameters),
       baggage,
@@ -1447,18 +1447,26 @@ function build(
     } else if (delivery[0] === 'stopVat') {
       return meterControl.runWithoutMeteringAsync(stopVat);
     } else {
+      let complete = false;
       // Start user code running, record any internal liveslots errors. We do
       // *not* directly wait for the userspace function to complete, nor for
       // any promise it returns to fire.
       const p = Promise.resolve(delivery).then(unmeteredDispatch);
-
-      // Instead, we wait for userspace to become idle by draining the promise
-      // queue. We clean up and then return 'p' so any bugs in liveslots that
-      // cause an error during unmeteredDispatch will be reported to the
-      // supervisor (but only after userspace is idle).
+      p.finally(() => (complete = true));
+
+      // Instead, we wait for userspace to become idle by draining the
+      // promise queue. We clean up and then examine/return 'p' so any
+      // bugs in liveslots that cause an error during
+      // unmeteredDispatch (or a 'buildRootObject' that fails to
+      // complete in time) will be reported to the supervisor (but
+      // only after userspace is idle).
       return gcTools.waitUntilQuiescent().then(() => {
         afterDispatchActions();
-        return p;
+        // eslint-disable-next-line prefer-promise-reject-errors
+        return complete ? p : Promise.reject('buildRootObject unresolved');
+        // the only delivery that pays attention to a user-provided
+        // Promise is startVat, so the error message is specialized to
+        // the only user problem that could cause complete===false
       });
     }
   }
diff --git a/packages/SwingSet/src/supervisors/supervisor-helper.js b/packages/SwingSet/src/supervisors/supervisor-helper.js
index 00feba2b54d..b31788cd514 100644
--- a/packages/SwingSet/src/supervisors/supervisor-helper.js
+++ b/packages/SwingSet/src/supervisors/supervisor-helper.js
@@ -37,7 +37,7 @@ function makeSupervisorDispatch(dispatch) {
         err => {
           // TODO react more thoughtfully, maybe terminate the vat
           console.log(`error ${err} during vat dispatch() of ${delivery}`);
-          return harden(['error', `${err.message}`, null]);
+          return harden(['error', `${err}`, null]);
         },
       );
   }
diff --git a/packages/SwingSet/test/upgrade/vat-ulrik-2.js b/packages/SwingSet/test/upgrade/vat-ulrik-2.js
index 1ce8da452c1..c4a14059f74 100644
--- a/packages/SwingSet/test/upgrade/vat-ulrik-2.js
+++ b/packages/SwingSet/test/upgrade/vat-ulrik-2.js
@@ -18,7 +18,7 @@ export const buildRootObject = (_vatPowers, vatParameters, baggage) => {
   const durandalHandle = baggage.get('durandalHandle');
   defineDurableKind(durandalHandle, initialize, behavior);
 
-  return Far('root', {
+  const root = Far('root', {
     getVersion: () => 'v2',
     getParameters: () => vatParameters,
     getPresence: () => baggage.get('presence'),
@@ -42,4 +42,6 @@ export const buildRootObject = (_vatPowers, vatParameters, baggage) => {
       return { imp33, imp35, imp37, imp38 };
     },
   });
+  // exercise async return
+  return Promise.resolve(root);
 };
diff --git a/packages/SwingSet/test/vat-admin/broken-hang-vat.js b/packages/SwingSet/test/vat-admin/broken-hang-vat.js
new file mode 100644
index 00000000000..01d1b56d8f0
--- /dev/null
+++ b/packages/SwingSet/test/vat-admin/broken-hang-vat.js
@@ -0,0 +1,6 @@
+import { makePromiseKit } from '@endo/promise-kit';
+
+export function buildRootObject() {
+  const pk = makePromiseKit();
+  return pk.promise; // never resolves
+}
diff --git a/packages/SwingSet/test/vat-admin/new-vat-13.js b/packages/SwingSet/test/vat-admin/new-vat-13.js
index 45a60dbc84c..53d559a961d 100644
--- a/packages/SwingSet/test/vat-admin/new-vat-13.js
+++ b/packages/SwingSet/test/vat-admin/new-vat-13.js
@@ -20,7 +20,7 @@ export function buildRootObject(_vatPowers, vatParameters) {
       },
     });
   }
-  return Far('root', {
+  const root = Far('root', {
     getANumber() {
       return 13;
     },
@@ -31,4 +31,6 @@ export function buildRootObject(_vatPowers, vatParameters) {
       return rcvrMaker(init);
     },
   });
+  // exercise async return
+  return Promise.resolve(root);
 }
diff --git a/packages/SwingSet/test/vat-admin/test-create-vat.js b/packages/SwingSet/test/vat-admin/test-create-vat.js
index 930cae09fd8..cd95b285003 100644
--- a/packages/SwingSet/test/vat-admin/test-create-vat.js
+++ b/packages/SwingSet/test/vat-admin/test-create-vat.js
@@ -30,6 +30,9 @@ test.before(async t => {
   const brokenRootVatBundle = await bundleSource(
     new URL('broken-root-vat.js', import.meta.url).pathname,
   );
+  const brokenHangVatBundle = await bundleSource(
+    new URL('broken-hang-vat.js', import.meta.url).pathname,
+  );
   const vatRefcountBundle = await bundleSource(
     new URL('new-vat-refcount.js', import.meta.url).pathname,
   );
@@ -39,6 +42,7 @@ test.before(async t => {
     vat44Bundle,
     brokenModuleVatBundle,
     brokenRootVatBundle,
+    brokenHangVatBundle,
     vatRefcountBundle,
     nonBundle,
   };
@@ -53,6 +57,7 @@ async function doTestSetup(t, enableSlog = false) {
     new13: { bundle: bundles.vat13Bundle },
     brokenModule: { bundle: bundles.brokenModuleVatBundle },
     brokenRoot: { bundle: bundles.brokenRootVatBundle },
+    brokenHang: { bundle: bundles.brokenHangVatBundle },
   };
   const hostStorage = provideHostStorage();
   await initializeSwingset(config, [], hostStorage, { kernelBundles });
@@ -123,7 +128,7 @@ test('counter test', async t => {
   t.deepEqual(JSON.parse(c.kpResolution(kpid).body), [4, 9, 2, 8]);
 });
 
-async function brokenVatTest(t, bundleName) {
+async function brokenVatTest(t, bundleName, expected) {
   const { c } = await doTestSetup(t);
   const kpid = c.queueToVatRoot(
     'bootstrap',
@@ -136,15 +141,19 @@ async function brokenVatTest(t, bundleName) {
   t.truthy(res instanceof Error);
   // 'Vat Creation Error: Error: missing is not defined'
   t.regex(res.message, /Vat Creation Error/);
-  t.regex(res.message, /missing/);
+  t.regex(res.message, expected);
 }
 
 test('broken vat creation fails (bad module)', async t => {
-  await brokenVatTest(t, 'brokenModule');
+  await brokenVatTest(t, 'brokenModule', /missing/);
 });
 
 test('broken vat creation fails (bad buildRootObject)', async t => {
-  await brokenVatTest(t, 'brokenRoot');
+  await brokenVatTest(t, 'brokenRoot', /missing/);
+});
+
+test('broken vat creation fails (buildRootObject hangs)', async t => {
+  await brokenVatTest(t, 'brokenHang', /buildRootObject unresolved/);
 });
 
 test('error creating vat from non-bundle', async t => {