I think I found out what problem is. I had the same problem with the gamerule command. It kept telling me that I had an incorrect argument. At first I didn't understand, but then I noticed that the new auto tab complete system is wrong. if you type: "/gamerule dod" and press tab, it completes the command like this: "/gamerule dodaylightcycle" when it has to be: "/gamerule doDayLightCycle" instead. The problem is that the auto tab complete doesn't mind upper case letters, while the command execution does.
I think I found out what problem is. I had the same problem with the gamerule command. It kept telling me that I had an incorrect argument. At first I didn't understand, but then I noticed that the new auto tab complete system is wrong. if you type: "/gamerule dod" and press tab, it completes the command like this: "/gamerule dodaylightcycle" when it has to be: "/gamerule doDayLightCycle" instead. The problem is that the auto tab complete doesn't mind upper case letters, while the command execution does.
I hope that this helpes,
Lenardjee