Checkbox validation not working

I have a required checkbox for data privacy. If the checkbox is not checked on submission I get a message like

Verification failed
Privacy value is required

on a blank WP Error Page.

// Validate Privacy Policy acceptance
function theme_af_validate_acceptance( $form, $args ) {
	$acceptance = af_get_field( 'acceptance_privacy_policy' );
	if ( $acceptance !== true ) {
		af_add_error( 'acceptance_privacy_policy', __('You must accept the privacy policy', 'text-domain') );
	}
}
add_action( 'af/form/validate', 'theme_af_validate_acceptance', 10, 2 );

What am I doing wrong?