Generating an app mlappinstall file

I am attempting to programmatically create a mlappinstall file (to install an app) as follows :

matlab.apputil.package( 'Z:\App Files\app_project_file.prj' )

I can see the project file exists in the correct location (and contains the required information) but when I issue the above command no mlappinstall file is generated (anywhere) yet no error or warning is generated.

Yet if I use

matlab.apputil.create( Z:\App Files\app_project_file.prj' )

and build, everything works fine.

Am I missing something ? Can I get error messages returned from matlab.apputil.package ?