MEncoder outputs "could not find matching colorspace"

[codemaster]

When I start to encode a DVD title to H.264 using MEncoder and x264, MEncoder outputs the following:

Could not find matching colorspace - retrying with -vf scale...
Opening video filter: [scale]
The selected video_out device is incompatible with this codec.
Try appending the scale filter to your filter list,
e.g. -vf spp,scale instead of -vf spp.

but it continues to encode without any problem and the encoded stream looks properly encoded.

Theese are the parameters I use in MEncoder for the first pass:

mencoder vobfile.vob -vf crop=704:464:12:8,yadif,hqdn3d -nosound -ovc x264 -x264encopts threads=auto:pass=1:profile=high:bitrate=1220:preset=slower -of rawvideo -o /dev/null

and the second pass:

mencoder vobfile.vob -vf crop=704:464:12:8,yadif,hqdn3d -nosound -ovc x264 -x264encopts threads=auto:pass=2:profile=high:bitrate=1220:preset=slower -of rawvideo -o encodedstream.264

But even if I append the scale filter to the above parameters, like MEncoder tells me:

mencoder vobfile.vob -vf crop=704:464:12:8,yadif,hqdn3d,scale

then MEncoder still outputs the same message as it does when I don't append the scale filter to the filter list:

Could not find matching colorspace - retrying with -vf scale...
Opening video filter: [scale]
The selected video_out device is incompatible with this codec.
Try appending the scale filter to your filter list,
e.g. -vf spp,scale instead of -vf spp.

This is what the scale filter does (it's from mencoder's manual):

scale[=w:h[:interlaced[:chr_drop[:par[:par2[:presize[:noup[:arnd]]]]]]]]

Scales the image with the software scaler (slow) and performs a YUV<−>RGB colorspace conversion

From this, I understand that it converts the source frames to RBG before sending them to x264, but I don't know if this conversion is necesary, therefore, I don't know if the scale filter is needed, but even if I add it, mencoder tells me to add it.