add helpful error when initial data is a non-object

packages/swingset-liveslots/src/virtualObjectManager.js

@@ -881,6 +881,10 @@ export function makeVirtualObjectManager(
       // kdebug(`vo make ${baseRef}`);
 
       const initialData = init ? init(...args) : {};
+      if (typeof initialData !== 'object') {
+        Fail`initial data must be object, not ${initialData}`;
+        // a common mistake is to use `() => {foo:1}`, not `() => ({foo:1})`
+      }
 
       // save (i.e. populate the cache) with the initial serialized record
       const capdatas = {};

packages/swingset-liveslots/test/virtual-objects/test-empty-data.js

@@ -0,0 +1,34 @@
+import test from 'ava';
+import '@endo/init/debug.js';
+
+import { makeFakeVirtualObjectManager } from '../../tools/fakeVirtualSupport.js';
+
+test('non-object initial data message', async t => {
+  const vom = makeFakeVirtualObjectManager();
+  // 'goodInit' is () => ({ value: 0 })
+  const goodInit = () => ({ value: 0 });
+
+  // 'badInit' is  () =>  { value: 0 }
+  //
+  // badInit is subtly wrong: it returns undefined, and includes an
+  // unused "value:" label, plus a side-effect-free evaluation of a 0
+  // constant. A linter might catch this, but maybe you didn't run
+  // one.
+  //
+  // I frequently mean to write goodInit and write badInit
+  // instead. It's such an easy mistake to make, so the VOM's instance
+  // maker will catch it and complain.
+  //
+  // note: it is not easy to convince our editors and linters and
+  // prettiers to leave this erroneous code in place
+
+  // eslint-disable-next-line
+  /* prettier-ignore */ const badInit = () => { value: 0 };
+
+  const behavior = {};
+  const makeGoodThing = vom.defineKind('goodthing', goodInit, behavior);
+  const makeBadThing = vom.defineKind('badthing', badInit, behavior);
+  makeGoodThing();
+  const m = s => ({ message: s });
+  t.throws(() => makeBadThing(), m(/initial data must be object, not /));
+});