Using exec on project with no git_url/hg_url is causing warning

Using exec('something') in a project which doesn't have a git_url or hg_url is causing the following warnings:

Use of uninitialized value $res_name in hash element at rbm/lib/RBM.pm line 579.
Use of uninitialized value $res_name in hash element at rbm/lib/RBM.pm line 580.