A stopper of `false` raises an error

If the factory returns `false` Xapian raises the following error:

ArgumentError: Wrong arguments for overloaded method 'QueryParser.stopper='.
Possible C/C++ prototypes are:
    void QueryParser.stopper=(Xapian::Stopper const *stop)
    void QueryParser.stopper=()
from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/gems/xapian-fu-1.5.0/lib/xapian_fu/query_parser.rb:100:in `stopper='
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/gems/xapian-fu-1.5.0/lib/xapian_fu/query_parser.rb:100:in `query_parser'
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/gems/xapian-fu-1.5.0/lib/xapian_fu/query_parser.rb:84:in `parse_query'
	from /Users/garry/.rvm/gems/ruby-2.0.0-p247@mediagreenhouse/gems/xapian-fu-1.5.0/lib/xapian_fu/xapian_db.rb:258:in `search'