picamera error the second time I run camera.capture(output,'yuv')

Question

I'm getting an error the second time I run camera.capture(output,'yuv'), what am I doing wrong?

my code:

from time import sleep
from picamera import PiCamera
from picamera.array import PiYUVArray
import numpy
w=64
h=48
camera=PiCamera()
output=PiYUVArray(camera)
camera.resolution=(w,h)
camera.start_preview()
sleep(2)
while True:
    print('output')
    camera.capture(output,'yuv')
    #some elaboration here
    sleep(10)

output:

output
output
Traceback (most recent call last):
  File "test1.py", line 14, in <module>
    camera.capture(output,'yuv')
  File "/usr/lib/python2.7/dist-packages/picamera/camera.py", line 1421, in capture
    if not encoder.wait(self.CAPTURE_TIMEOUT):
  File "/usr/lib/python2.7/dist-packages/picamera/encoders.py", line 395, in wait
    self.stop()
  File "/usr/lib/python2.7/dist-packages/picamera/encoders.py", line 419, in stop
    self._close_output()
  File "/usr/lib/python2.7/dist-packages/picamera/encoders.py", line 349, in _close_output
    mo.close_stream(output, opened)
  File "/usr/lib/python2.7/dist-packages/picamera/mmalobj.py", line 371, in close_stream
    stream.flush()
  File "/usr/lib/python2.7/dist-packages/picamera/array.py", line 296, in flush
    self.array = bytes_to_yuv(self.getvalue(), self.size or self.camera.resolution)
  File "/usr/lib/python2.7/dist-packages/picamera/array.py", line 93, in bytes_to_yuv
    'Incorrect buffer length for resolution %dx%d' % (width, height))
picamera.exc.PiCameraValueError: Incorrect buffer length for resolution 64x48