'@r' target selector in scoreboard commands ignores "not equal to" arguments about entity type

I was trying to use scoreboard commands to give a randomly selected entity that isn't a chicken a certain score in an objective when I found that the command was only selecting players. This is the command I was using:

/scoreboard players set @r[type=!chicken] Class 1

As shown in the first screenshot, despite there being many targets that aren't players, I still could never get a non-player entity to be selected.
This is strange because "equal to" arguments still work, as shown in the second screenshot.

Also, other commands don't seem to have this issue. /testfor works fine with this type of selector, as shown in the third screenshot.