If you don't provide any syntax, no syntax error will occur

When I do /gamemode with no argument, the parser says that it is an "unknown command".
How to reproduce
Go into any world on 17w46a
Write /gamemode
Look as you will get an command error, not a syntax error
What I expected to happen
...was that I would get "Invalid usage: usage /gamemode <creative|survival|adventure|spectator>" or something simular. At least "Syntax error: argument 0 may not be undefined", but I get "unknown command"