diff --git a/shared/diy.js b/shared/diy.js
index 23de7849..c3cb24bc 100644
--- a/shared/diy.js
+++ b/shared/diy.js
@@ -146,8 +146,9 @@ export function checkDiyPick(slotId, pick, data) {
   if (proto) {
     const lk = lockedSelection(data, slot.tier, charId);
     if (!lk) return { error: `${charId}: no locked selection at tier ${slot.tier}` };
+    // omitted / null = its locked selection; another skill, another module or an explicit 'none' is refused
     if (want !== null && want !== lk.skillIndex) return { error: `${charId}: a prototype carries skill ${lk.skillIndex} (与系统补位时一致)` };
-    if (pick.uniEquipId !== undefined && wantMod !== lk.uniEquipId) return { error: `${charId}: a prototype carries module ${lk.uniEquipId ?? 'none'}` };
+    if (pick.uniEquipId != null && wantMod !== lk.uniEquipId) return { error: `${charId}: a prototype carries module ${lk.uniEquipId ?? 'none'}` };
     ({ skillIndex, uniEquipId } = lk);
   } else {
     if (want === null) return { error: `${charId}: an owned pick needs a skill index` };
diff --git a/test/content/kits_layout.test.js b/test/content/kits_layout.test.js
index 77f97ff0..0b073d7b 100644
--- a/test/content/kits_layout.test.js
+++ b/test/content/kits_layout.test.js
@@ -14,6 +14,8 @@ import { join, dirname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { KIT_FILES, TIER_KITS, KITS, STANDIN_KIT_FILES, STANDIN_KITS, OPERATOR_KIT_FILES, OPERATOR_KITS, GENERIC_KIT_CHARS, KITTED_CHARS } from '../../server/sim/content/kits/index.js';
 import { KITS as CONTENT_KITS } from '../../server/sim/content/index.js';
+import { diyRecord } from '../../shared/diy.js';
+import { normalizeChess } from '../../server/sim/simdata.js';
 
 const ROOT = join(dirname(fileURLToPath(import.meta.url)), '../..');
 const KIT_DIR = join(ROOT, 'server/sim/content/kits');
@@ -54,10 +56,15 @@ test('a 自选 kit file op-<codename>.js registers the kit of that owned-6★ pi
     assert.equal(typeof mod.default[charId], 'function', `${file}: kit builder`);
     assert.equal(OPERATOR_KITS[charId], mod.default[charId], `${file}: registered`);
     assert.equal(KITS[charId], mod.default[charId], `${file}: in the merged registry`);
-    // every skill of every form is authored (a 自选 piece has no default skill: the pick chooses any)
-    for (const [key, form] of Object.entries(BACKUPS.units[charId].forms)) {
-      const kit = mod.default[charId](form.skills[0].bb, { ...form, skill: form.skills[0], charId }, {});
-      assert.deepEqual(Object.keys(kit.skills || {}).sort(), form.skills.map((s) => s.skillId).sort(), `${file}@${key}: skills`);
+    // every skill of every slot form is authored (a 自选 piece has no default skill: the pick chooses any), built from the
+    // composed record and its def as the sim does (shared/diy.js diyRecord, simdata normalizeChess)
+    for (const slot of ['chess_char_5_diy1_a', 'chess_char_6_diy1_a']) {
+      for (const elite of [false, true]) {
+        const rec = diyRecord(slot, { charId, skillIndex: 0 }, { elite, data: { chess: CHESS, backups: BACKUPS } });
+        assert.ok(rec, `${file}: a legal pick of ${slot}`);
+        const kit = mod.default[charId](rec.skill.bb, rec, normalizeChess(rec));
+        assert.deepEqual(Object.keys(kit.skills || {}).sort(), rec.skills.map((s) => s.skillId).sort(), `${file}@${slot}${elite ? ' elite' : ''}: skills`);
+      }
     }
   }
   assert.deepEqual([...KITTED_CHARS].sort(), [...new Set([...GENERIC_KIT_CHARS, ...Object.keys(STANDIN_KITS), ...Object.keys(OPERATOR_KITS)])].sort());
diff --git a/test/diy.test.js b/test/diy.test.js
index cccb32cb..a9e141f0 100644
--- a/test/diy.test.js
+++ b/test/diy.test.js
@@ -70,6 +70,8 @@ test('checkDiyPick: a prototype carries its locked skill / module (与系统补
   assert.deepEqual(lockedSelection(DATA, 6, 'char_617_sharp2'), { skillIndex: 0, uniEquipId: 'uniequip_002_sharp2' });
   assert.deepEqual(lockedSelection(DATA, 5, 'char_605_cmedic'), { skillIndex: 2, uniEquipId: null });
   assert.match(checkDiyPick(T5, { charId: 'char_609_acguad', uniEquipId: 'none' }, DATA).error, /prototype carries module/);
+  assert.match(checkDiyPick(T5, { charId: 'char_609_acguad', uniEquipId: 'uniequip_001_acguad' }, DATA).error, /prototype carries module/);
+  assert.deepEqual(checkDiyPick(T5, { charId: 'char_609_acguad', skillIndex: null, uniEquipId: null }, DATA).pick, { charId: 'char_609_acguad', skillIndex: 2, uniEquipId: 'uniequip_002_acguad' }, 'null = the locked selection');
   assert.equal(lockedSelection(DATA, 5, SIEGE), null, 'an owned operator has no lock');
 });