IO error in Kivy

ioerror.py

# Encoding: UTF-8

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.label import Label
from kivy.clock import Clock
from kivy.animation import Animation

class CountWidget(Widget):
    def __init__(self):
        super(CountWidget, self).__init__()
        Clock.schedule_once(lambda dt: self.count(), 1)

    def count(self, num=0):
        label = Label(
                text=str(num),
                font_size=24,
                color=(0, 0, 1, 1),
            )
        animation = Animation(font_size=256, color=(0, 0, 1, 0), duration=3)
        animation.start(label)
        self.add_widget(label)
        Clock.schedule_once(lambda dt: self.count(num + 1), 0.5)

class TheApp(App):
    def build(self):
        return CountWidget()

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

output

$ python ioerror.py 
[INFO   ] Kivy v1.0.7-dev
[INFO   ] [Logger      ] Record log in /home/petr/.kivy/logs/kivy_11-06-05_93.txt
[INFO   ] [Factory     ] 93 symbols loaded
[INFO   ] [Text        ] using <pygame> as text provider
[INFO   ] [Window      ] using <pygame> as window provider
[WARNING] [WinPygame   ] Video: failed (multisamples=2)
[WARNING] [WinPygame   ] trying without antialiasing
[INFO   ] [GL          ] OpenGL version <2.1 Mesa 7.9-devel>
[INFO   ] [GL          ] OpenGL vendor <Tungsten Graphics, Inc>
[INFO   ] [GL          ] OpenGL renderer <Mesa DRI Intel(R) Ironlake Mobile GEM 20100330 DEVELOPMENT >
[INFO   ] [GL          ] Shading version <1.20>
[INFO   ] [GL          ] Texture max size <4096>
[INFO   ] [GL          ] Texture max units <16>
[INFO   ] [Shader      ] fragment compiled successfully
[INFO   ] [Shader      ] vertex compiled successfully
[INFO   ] [OSC         ] using <multiprocessing> for socket
[INFO   ] [Base        ] Start application main loop
[INFO   ] [OSC         ] listening for Tuio on 0.0.0.0:3333
[INFO   ] [Base        ] Leaving application in progress...
 Traceback (most recent call last):
   File "ioerror.py", line 30, in <module>
     TheApp().run()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/app.py", line 197, in run
     runTouchApp()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/base.py", line 401, in runTouchApp
     EventLoop.window.mainloop()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/window/window_pygame.py", line 254, in mainloop
     self._mainloop()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/window/window_pygame.py", line 173, in _mainloop
     EventLoop.idle()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/base.py", line 265, in idle
     Clock.tick()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/clock.py", line 251, in tick
     self._process_events()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/clock.py", line 356, in _process_events
     if event.tick(self._last_tick) is False:
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/clock.py", line 170, in tick
     ret = callback(self._dt)
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/uix/label.py", line 81, in texture_update
     self._label.refresh()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/text/__init__.py", line 334, in refresh
     sz = self.render()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/text/__init__.py", line 190, in render
     lw, lh = self.get_extents(line)
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/text/text_pygame.py", line 59, in get_extents
     font = self._get_font()
   File "/home/petr/programs/virtualenv/mt/lib/python2.6/site-packages/kivy/core/text/text_pygame.py", line 38, in _get_font
     int(self.options['font_size'] * 1.333))
 IOError: unable to read font filename