diff --git a/packages/spec/scripts/lib/sharded-artifacts.ts b/packages/spec/scripts/lib/sharded-artifacts.ts
index 9f980415ba..2d213b1d53 100644
--- a/packages/spec/scripts/lib/sharded-artifacts.ts
+++ b/packages/spec/scripts/lib/sharded-artifacts.ts
@@ -340,6 +340,18 @@ function shardTextsByCategory(
   return out;
 }
 
+/**
+ * The JSON type of a parsed value, article included, for a message that has to
+ * tell an author what they actually wrote. `typeof` alone answers `"object"` for
+ * both `null` and `[]` — the two hand-edit accidents most worth telling apart.
+ */
+function jsonTypeLabel(value: unknown): string {
+  if (value === null) return 'null';
+  if (Array.isArray(value)) return 'an array';
+  const type = typeof value;
+  return `${/^[aeiou]/.test(type) ? 'an' : 'a'} ${type}`;
+}
+
 /**
  * Aggregate a category-sharded directory into one sorted array, validating that
  * each shard answers only for its own category.
@@ -370,7 +382,23 @@ export function aggregateCategoryShards(
     if (!Array.isArray(list)) {
       throw new Error(`${dirName}/${shard.name}.json has no "${field}" array (#5837).`);
     }
-    for (const raw of list as string[]) {
+    // `Array.isArray` says the field IS an array and nothing about what is in
+    // it, so this is where untyped JSON stops being untyped. A hand-edited
+    // non-string entry used to reach `categoryOfDefKey`, whose parameter is
+    // declared `string`, and die there on `key.indexOf is not a function` —
+    // right exit code, but all three call sites print `error.message` alone, so
+    // the author was told neither the shard file nor the entry (#6751). Every
+    // other defect class this reader rejects names both; so does this one now.
+    // The check belongs here rather than in `categoryOfDefKey`: the helper's
+    // contract already says `string`, and only its caller knows the file name.
+    for (const [index, raw] of (list as readonly unknown[]).entries()) {
+      if (typeof raw !== 'string') {
+        throw new Error(
+          `${dirName}/${shard.name}.json ${field}[${index}] is ${jsonTypeLabel(raw)}, not a string ` +
+            `(#5837): ${JSON.stringify(raw)}. Every entry is a "<category>/<Def>[:<prop>]" key — ` +
+            `regenerate rather than reconcile by hand.`,
+        );
+      }
       if (categoryOfDefKey(raw) !== shard.name) {
         throw new Error(
           `${dirName}/${shard.name}.json carries "${raw}", which belongs to category ` +
diff --git a/packages/spec/scripts/sharded-artifacts.test.ts b/packages/spec/scripts/sharded-artifacts.test.ts
index b922542efb..b62f1db095 100644
--- a/packages/spec/scripts/sharded-artifacts.test.ts
+++ b/packages/spec/scripts/sharded-artifacts.test.ts
@@ -87,6 +87,33 @@ function changed(before: Map<string, string>, after: Map<string, string>): strin
   return [...names].filter((n) => before.get(n) !== after.get(n)).sort();
 }
 
+/**
+ * The message of the error `run` throws, for the cases where the MESSAGE is the
+ * thing under test (#6751). `expect(...).toThrow()` cannot serve those: the
+ * reader threw before the fix too — with a bare `TypeError` — so a throw-only
+ * assertion is green on the defect it is supposed to pin.
+ */
+function messageOf(run: () => unknown): string {
+  try {
+    run();
+  } catch (error) {
+    return error instanceof Error ? error.message : String(error);
+  }
+  return expect.fail('expected the shard reader to reject, but it returned a value');
+}
+
+/** Rewrite one shard's parsed document, in the canonical byte shape. */
+function rewriteShard(
+  root: string,
+  name: string,
+  mutate: (doc: Record<string, unknown>) => void,
+): void {
+  const file = path.join(root, `${name}.json`);
+  const doc = JSON.parse(fs.readFileSync(file, 'utf-8'));
+  mutate(doc);
+  fs.writeFileSync(file, JSON.stringify(doc, null, 2) + '\n');
+}
+
 describe('sharded artifacts — locality: a change in one category moves one file (#5837)', () => {
   it('routes a key to the shard its category segment names, and only that one', () => {
     // The routing law itself. Everything below is a consequence of it, so it is
@@ -229,6 +256,79 @@ describe('sharded artifacts — the aggregate reads the whole directory (#5837)'
     expect(() => aggregateCategoryShards(dir, 'keys')).toThrow(/declares category "data"/);
   });
 
+  /**
+   * The fourth defect class of this reader (#6751). The other three name the
+   * shard file and carry an issue anchor; a non-string entry used to reach
+   * `categoryOfDefKey` — whose parameter is declared `string` — and die on
+   * `key.indexOf is not a function`. All three call sites in `build-schemas.ts`
+   * print `error.message` and nothing else, so that bare text WAS the whole
+   * diagnostic: no file among 14 shards, no entry, no anchor.
+   *
+   * Note what these cases may not do: assert only that it throws. The unfixed
+   * reader throws too, so `expect(...).toThrow()` — and even `toThrow(/./)` —
+   * stays green on exactly the defect. The message is the contract here, so the
+   * file, the entry and the anchor are each pinned by name.
+   */
+  it('names the shard file, the entry index and the anchor when an entry is not a string', () => {
+    writeShards(dir, authorableSurfaceShardTexts(KEYS));
+    rewriteShard(dir, 'ui', (doc) => {
+      (doc.keys as unknown[])[0] = 12345;
+    });
+
+    const message = messageOf(() => aggregateCategoryShards(dir, 'keys'));
+    expect(message, 'names the shard file').toContain('ui.json');
+    expect(message, 'names the entry').toContain('keys[0]');
+    expect(message, 'carries the issue anchor').toContain('#5837');
+    expect(message, 'says what was found instead').toContain('is a number, not a string');
+    expect(message, 'quotes the offending value').toContain('12345');
+    // The regression this exists for, stated as itself.
+    expect(message, 'never the bare JS error again').not.toContain('indexOf');
+  });
+
+  it('distinguishes null and object entries, which `typeof` alone reports as one', () => {
+    // `typeof null === 'object'` is the trap the type label exists for: telling
+    // an author "object" when they wrote `null` sends them looking for a brace.
+    writeShards(dir, authorableSurfaceShardTexts(KEYS));
+    rewriteShard(dir, 'ui', (doc) => {
+      (doc.keys as unknown[])[1] = null;
+    });
+    expect(messageOf(() => aggregateCategoryShards(dir, 'keys'))).toContain(
+      'keys[1] is null, not a string',
+    );
+
+    writeShards(dir, authorableSurfaceShardTexts(KEYS));
+    rewriteShard(dir, 'ui', (doc) => {
+      (doc.keys as unknown[])[1] = { 'ui/View:name': true };
+    });
+    expect(messageOf(() => aggregateCategoryShards(dir, 'keys'))).toContain(
+      'keys[1] is an object, not a string',
+    );
+  });
+
+  it('speaks the same way for every sharded artifact, naming that artifact’s own field', () => {
+    // `categoryOfDefKey` is shared by all three category-sharded ratchets
+    // (authorable-surface/, json-schema.manifest/, authorable-defaults/), so the
+    // dumb message appeared under three different prefixes. The field name is
+    // part of the message for the same reason the file name is.
+    //
+    // The entry here is an ARRAY on purpose, and it is the worst of the class:
+    // `['ui/View'].indexOf('/')` is a perfectly valid Array.prototype call that
+    // answers -1, so this case never produced a TypeError at all — it fell
+    // through to `cannot shard "ui/View": … has no category segment`, naming a
+    // key that is not in the file and a cause that is not the defect. A wrong
+    // diagnosis outranks a bare one, which is why the type is checked before
+    // the routing rather than left to whatever `indexOf` happens to mean.
+    writeShards(dir, schemaManifestShardTexts(['ai/Agent', 'ui/View', 'ui/Dashboard']));
+    rewriteShard(dir, 'ui', (doc) => {
+      (doc.schemas as unknown[])[1] = ['ui/View'];
+    });
+
+    const message = messageOf(() => aggregateCategoryShards(dir, 'schemas'));
+    expect(message).toContain('ui.json');
+    expect(message).toContain('schemas[1] is an array, not a string');
+    expect(message).toContain('#5837');
+  });
+
   it('refuses a stray file in a generator-owned directory', () => {
     writeShards(dir, authorableSurfaceShardTexts(KEYS));
     fs.writeFileSync(path.join(dir, 'notes.txt'), 'scratch\n');