diff --git a/packages/downgrader/src/shared.ts b/packages/downgrader/src/shared.ts
index ef2eccf..8667f62 100644
--- a/packages/downgrader/src/shared.ts
+++ b/packages/downgrader/src/shared.ts
@@ -153,14 +153,14 @@ export function map(convert: Convert, isEntry: (key: string) => boolean = () =>
     if (!isRecord(value)) {
       return clone(value)
     }
-    const out: [string, unknown][] = []
+    const out: Record<string, unknown> = {}
     for (const [key, item] of Object.entries(value)) {
       const converted = item === undefined ? DROP : isEntry(key) ? convert(item, ctx) : clone(item)
       if (converted !== DROP) {
-        out.push([key, converted])
+        setOwn(out, key, converted)
       }
     }
-    return Object.fromEntries(out)
+    return out
   }
 }
 
diff --git a/packages/downgrader/tests/v3.1-to-v3.0.test.ts b/packages/downgrader/tests/v3.1-to-v3.0.test.ts
index d449cc3..a91c42d 100644
--- a/packages/downgrader/tests/v3.1-to-v3.0.test.ts
+++ b/packages/downgrader/tests/v3.1-to-v3.0.test.ts
@@ -1293,13 +1293,15 @@ describe('unusual input', () => {
   it('keeps own __proto__ keys as keys', () => {
     const out = downgradeSchemaV31ToV30(
       JSON.parse(
-        '{"type":"object","properties":{"__proto__":{"type":["string","null"]}},"example":{"__proto__":1}}',
+        '{"type":"object","properties":{"__proto__":{"type":["string","null"]},"1":{}},"example":{"__proto__":1}}',
       ),
     ) as Record<string, any>
     expect(Object.getOwnPropertyDescriptor(out.properties, '__proto__')?.value).toEqual({
       type: 'string',
       nullable: true,
     })
+    expect(Object.keys(out.properties)).toEqual(['1', '__proto__'])
+    expect(Object.getPrototypeOf(out.properties)).toBe(Object.prototype)
     expect(Object.getOwnPropertyDescriptor(out.example, '__proto__')?.value).toBe(1)
     expect(Object.getPrototypeOf(out.example)).toBe(Object.prototype)
   })