Why can't I assign to var in Scala subclass?

Suppose I have the following abstract class:

abstract class A (var is_happy : Boolean) {
  def toggle_happiness();
}

And now I want to define a concrete class which implements the toggle_happiness() method:

class B (is_happy : Boolean) extends A (is_happy) {
  def toggle_happiness() = {
    is_happy = !is_happy
  }
}

Scala's compiler gives me:

error: reassignment to val
   is_happy = !is_happy
            ^

What's going on here? I thought that is_happy referred to a var in my class that is set by my constructor. Do I have a conflict with the name is_happy?

Thanks, Dan