kivy imagebutton

Gives me "Invalid class name" on <ImageButton> .kv line
#.py

from kivy.uix.behaviors import ButtonBehavior
#..

class ImageButton(ButtonBehavior, Image):
    pass


class StepWidget(GridLayout):

    def __init__(self, **kwargs):
        super(StepWidget, self).__init__(**kwargs)
        #...
    def pressed(self):
        print('hooooray')

#.kv
# button behavior example: http://stackoverflow.com/questions/24668317/kivy-clickable-image-that-replaced-a-button

<StepWidget>:
    size_hint_x: '1'
    cols: 1
    <ImageButton>:
        on_press: root.pressed()