xrandr: Failed to get size of gamma for output default

Slackware 14.2 x64 (guest VM)
VirtualBox (5.1.38) - no VirtualBox Guest Additions yet.

I get an error/warning when issue xrandr command:
Code:# xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 400, current 1024 x 768, maximum 1024 x 768
default connected 1024x768+0+0 0mm x 0mm
1024x768 61.00*
800x600 73.00
640x480 86.00
640x400 86.00I tried --addmode as some people reported to overcome this problem this way but in my case it doesnt help.
Any ideas what else to try?
Code:# xrandr --monitorlist
xrandr: Failed to get size of gamma for output default
Monitors: 1
0: +default 1024/271x768/203+0+0 default

# xrandr --addmode default 1024x768
xrandr: Failed to get size of gamma for output default