Kivy: kwargs ignored for properties set via kv

gistfile1.txt

import kivy
kivy.require('1.8.1')

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button

Builder.load_string('''
<Button>:
    text: 'bad'
''')

class TestApp(App):
    def build(self):
        return Button(text='good')

if __name__ == '__main__':
    TestApp().run()