How come not specifying the executable causes an error?

Hi

I'm trying to get CMake to build using an existing Makefile using the following command:

add_custom_target(make-all ALL COMMAND make -C ${mytest_SOURCE_DIR})

When I run make-all without selecting an executable, I'm always shown an error first:

The run can be forced, but why is it necessarily an error if an executable is not selected?

Thanks