Problem changing resolution using xrandr: BadMatch

I tried to change the resolution using the following commands (found here):

Use xrandr to check the current configuration and available modes:

$ xrandr
Screen 0: minimum 64 x 64, current 800 x 600, maximum 32000 x 32000
VBOX1 connected 800x600+0+0 0mm x 0mm
   800x600        60.0*+
   640x480        59.9  

Use cvt to get the proper modeline for 1280x800:

$ cvt 1280 800
# 1280x800 59.81 Hz (CVT 1.02MA) hsync: 49.70 kHz; pclk: 83.50 MHz
Modeline "1280x800_60.00"   83.50  1280 1352 1480 1680  800 803 809 831 -hsync +vsync

Create the new mode using xrandr --newmode:

$ xrandr --newmode "1280x800_60.00"   83.50  1280 1352 1480 1680  800 803 809 831 -hsync +vsync

Use xrandr again to check that the mode was created successfully:

$ xrandr
Screen 0: minimum 64 x 64, current 800 x 600, maximum 32000 x 32000
VBOX1 connected 800x600+0+0 0mm x 0mm
   800x600        60.0*+
   640x480        59.9  
  1280x800_60.00 (0x11a)   83.0MHz
        h: width  1280 start 1352 end 1480 total 1680 skew    0 clock   49.4KHz
        v: height  800 start  803 end  809 total  831           clock   59.5Hz

However, when I try to add the new mode to the display with xrandr --addmode I get the following error:

$ xrandr --addmode VBOX1 1280x800_60.00
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  150 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  18
  Current serial number in output stream:  19

Of course, if I try to set the new resolution for my current display it fails (the mode isn't associated to te display):

$ xrandr --output VBOX1 --mode 1280x800_60.00
xrandr: cannot find mode 1280x800_60.00