Dropdown Category menu problem

I am using dropdown category menu to display a list of subcategories of the parent category.When a user select none option i.e “select category” than it goes to http://www.mysite.com/?cat=-1.
I don’t know why it chooses category -1.
Page shown is also weird.If I remove “select category” from dropdown than for drop down for single category doesn’t work.Please help me to solve the problem.