Command syntax help not showing error message when selector without arguments is not applicable

The bug

When a command only allows a certain type of entities, e.g. players, and a selector without any arguments does not match, no error message is shown.

How to reproduce

1. Type the following

   /clear @e
   

   "@e" is red, but no error message is shown

2. Type the brackets for selectors arguments: @e[]
   Now an error message is shown explaining why the selector is not allowed