How do you get CTRL-] to pick up the '?' in a Ruby method name?

If I generate a tags file for some code with rtags, and then put my cursor in Vim over a method like "confirmed?" and press CTRL-], Vim fails to find the tag because Vim doesn't see the '?' as part of the keyword being l开发者_StackOverflow中文版ooked up. Is there a way to fix this?

:set iskeyword+=?