Emper is not usable as a meson subproject

I tried to use emper as a dependency through a meson warp.

[wrap-git]  
url = git@gitlab.cs.fau.de:i4/emper.git  
revision = head

But meson fails to build emper as a subproject with this message:

subprojects/emper/meson.build:12:0: ERROR: Function 'add_global_arguments' cannot be used in subprojects because there is no way to make that reliable.
Please only call this if is_subproject() returns false. Alternatively, define a variable that
contains your language-specific arguments and add it to the appropriate *_args kwarg in each target.