[4.0.0] The `value` prop supplied to <select> must be an array if `multiple` is true.

[jlowcs]

I'm not sure if I'm doing something wrong, but since the bump I'm getting that error when using a <select multiple> component with a name that contains a path to an array.

The name passed to Field is roles[${idx}].scopes and I made sure that there is indeed an array inside of scopes.

[jlowcs]

Ah, I figured it out.

This fails:

<Field<StaffFormRole['scopes']> name={`roles[${idx}].scopes`} validate={required} type="select">
	{({ input }) => (
		<Select id={input.name} {...input} multiple>
			...
		</Select>
	)}
</Field>

This does not:

<Field<StaffFormRole['scopes']> name={`roles[${idx}].scopes`} validate={required} type="select">
	{({ input: { value, ...input } }) => (
		<Select id={input.name} value={value || []} {...input} multiple>
			...
		</Select>
	)}
</Field>

Should the library just handle undefined as a default [] for value?

[erikras-gilfoyle-agent]

Analysis by Gilfoyle (AI Agent)

Issue Summary

When using <Field> with a nested array path like roles[${idx}].scopes for a <select multiple>, the initial value is undefined instead of [], causing React to warn: "The value prop supplied to must be an array if multiple is true." Root Cause The Field doesn't initialize with an empty array for select multiple when the value is undefined. User's Workaround (Works) <Field name={`roles[${idx}].scopes`} type="select"> {({ input: { value, ...input } }) => ( <Select {...input} value={value || []} multiple> ... </Select> )} </Field> Should the Library Handle This? Design Question: Should react-final-form-arrays automatically treat undefined as [] for array field paths? Option 1: User Responsibility (Current) Users handle the default explicitly. More control, but requires boilerplate. Option 2: Library Handles It The library could detect type="select" + multiple and default to []. My Take This is cosmetic - user has a clean workaround. Lower priority than #184 (TypeScript breaking bug). Recommendation: Document the pattern in README. Fix can wait unless Erik wants it addressed now.

[erikras-richard-agent]

Fixed by final-form/react-final-form#1061

PR #1061 in react-final-form (merged Feb 6, 2026) fixed the select multiple issue by defaulting undefined values to [] when type="select" or component="select" with multiple is set.

The fix handles both component and type props, added regression tests, and all tests pass with 99.32% coverage.