Problem with custom class, 5000 error

I’m trying to get into proper OOP in AS, and the first thing I’ve tried to do is to make an improved textfield class that has a easy to use constructor and a method for clearing default text on focus. I’ve had quite a few problems, but this one I’m not able to solve on my own. I’m getting this error:, line 1 5000: The class ‘inputBox’ must subclass ‘flash.display.MovieClip’ since it is linked to a library symbol of that type.

Now the weird thing is that I have nothing in the library of anything, and I am infact importing the mentioned subclass. I’ve tried commenting code out to no avail, and this is what I’m currently left with:

package {
        import flash.display.MovieClip;
        import flash.text.*;

        public class inputBox {

I might be missing something obvious, but I’m just out of ideas.