vp8enc: changing width or height while playing causes error

[stic at free.fr]

Hello gstreamer friends,

I would like to be able to change resolution (width or height) in caps given to vp8enc while the pipeline is playing, but it fails with error:
gstvideoencoder.c:608:gst_video_encoder_setcaps:<encoder> rejected caps video/x-raw, width=(int)640, height=(int)400, framerate=(fraction)20/1, format=(string)I420
gstvideoencoder.c:1368:gst_video_encoder_chain:<encoder> error: encoder not initialized

I don't have such error with other encoders, they support resolution change while it is playing.
Is it possible with vp8enc ?

Thanks a lot for your answers :)