Adding additional outputPaths throws merge-trees error

[knownasilya]

outputPaths: {
      app: {
        css: {
          'activate': '/assets/activate.css'
        }
      }
    },

Returns the following error:

Merge error: file assets/activate.css.map exists in /Users/iradchenko/workspace/dashboard/tmp/broccoli_merge_trees-input_base_path-Li6gifv7.tmp/0 and /Users/iradchenko/workspace/mapgeo2/dashboard/tmp/broccoli_merge_trees-input_base_path-Li6gifv7.tmp/1
Pass option { overwrite: true } to mergeTrees in order to have the latter file win.

And adding the overwrite to the merge trees in my ember-cli-build.js file doesn't do anything. Note, I do have an app.less.

[knownasilya]

@stefanpenner do you think this is a regression somewhere in the build pipeline?