fbset error

[tim_harris]

I have a Cyrix MediaGXm processor
I am running vesafb.
Unfortunately whenever I try and change the video mode I get this error:

ioctl: FBIOPUT_VSCREENINFO: Invalid argument

The same error occurs with the kgicon frame buffer.
Could someone please tell me what I am doing incorrectly?
Or at least what the error means.

Thanks,
Tim