Rails 3：如何防止我的验证从time_select下拉菜单中传递“nil time”？

I use time_select with a prompt ("Choose hours"...) and would like to make sure, the user actually enters a valid time. But if the form is submitted without a time being selected (so with the empty prompt options selected) I don't get any errors from my validations.I use time_select with a prompt ("Choose hours"