function isInvalidIndex( index ) {
	return !(
		isNumber( index ) ||
		isNull( index ) ||
		isUndefined( index )
	);
}
Tests whether a starting index argument is invalid.

@private
@param {*} index - index argument
@returns {boolean} boolean indicating if an argument is invalid

@example
var bool = isInvalidIndex( 4 );
// returns false

@example
var bool = isInvalidIndex( {} );
// returns true