Cannot read properties of undefined (reading 'constructor') at memoizedCreateChangeProxy when reading new nested property

[filipw01]

• I've validated the bug against the latest version of DB packages

Describe the bug
When reading a newly added nested property TypeError: Cannot read properties of undefined (reading 'constructor') is thrown

To Reproduce

import { createChangeProxy } from '@tanstack/db'

type Row = {
  id: string
  nested?: {
    count: number
  }
}

const original: Row = {
  id: '1',
}

const { proxy, getChanges } = createChangeProxy(original)

console.log('Initial changes:', getChanges())

// This writes to changeTracker.copy_.nested
proxy.nested = { count: 1 }

console.log('After assigning nested object:', getChanges())

try {
  console.log(proxy.nested?.count)
} catch (error) {
  console.error('Reproduced proxy bug:')
  console.error(error)
}

Expected behavior
In the example above 1 is returned

• OS: MacOS 26.3.1
• Browser: Chrome 146.0.7680.165

Additional context
I was updating lodash mergeWith to es-toolkit/compat and found that tanstack db started throwing when merging some objects behind its proxy

[KyleAMathews]

Resolved by #1797 (commit cfb01ce). The exact sequence—assign a previously absent nested object through the change proxy, call getChanges, then read the nested property—throws on the historical parent and passes on current main; the current proxy contract suite is 71/71 green. Closing as fixed, with the minimal witness retained as regression provenance. Thanks @filipw01 for the precise report.