Describe the bug

deep_set in form-utils.js throws TypeError: Cannot set properties of undefined when attempting to set a value at a nested path where an intermediate key exists on the object but has a value of undefined or null.

This happens because deep_set uses Object.hasOwn(current, key) to decide whether to auto-create an intermediate container. When a key exists but holds undefined, hasOwn returns true, so auto-creation is skipped. The function then does current = current[key] (which becomes undefined), and the next iteration tries to assign a property on undefined, which throws.

Reproduction

import { deep_set } from '@sveltejs/kit/src/runtime/form-utils.js';

const obj = { nested: undefined };

// This throws: TypeError: Cannot set properties of undefined (setting 'name')
deep_set(obj, ['nested', 'name'], 'hello');

Expected: deep_set auto-creates the intermediate object, resulting in { nested: { name: 'hello' } } — the same behavior as when the key is completely absent.

Actual: TypeError: Cannot set properties of undefined (setting 'name')

Real-world scenario

This can happen with RemoteForm when using optional nested objects in a Zod schema:

const schema = z.object({
    name: z.string(),
    foo: z.record(z.string(), z.string()).optional(),
});


form.fields.set({
   name: 'some-name',
   foo: undefined
});

// throws
form.fields.foo.bar.set('will fail')

Suggested fix

In deep_set (form-utils.js, line 517), also check whether the existing value is nullish:

- if (!exists) {
+ if (!exists || current[key] == null) {
    current[key] = is_array ? [] : {};
  }

Logs

System Info

System:
    OS: macOS 26.3.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 108.00 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.13.0 - /Users/hald/.nvm/versions/node/v24.13.0/bin/node
    npm: 11.6.2 - /Users/hald/.nvm/versions/node/v24.13.0/bin/npm
    pnpm: 10.31.0 - /Users/hald/.nvm/versions/node/v24.13.0/bin/pnpm
  Browsers:
    Chrome: 146.0.7680.154
    Edge: 146.0.3856.72
    Firefox: 137.0.2
    Safari: 26.3.1
  npmPackages:
    @sveltejs/adapter-vercel: ^6.3.3 => 6.3.3 
    @sveltejs/kit: ^2.55.0 => 2.55.0 
    @sveltejs/vite-plugin-svelte: ^6.1.1 => 6.2.1 
    svelte: ^5.54.0 => 5.54.0 
    vite: ^7.3.0 => 7.3.0

Severity

serious, but I can work around it

Additional Information

No response