I've got a polygon shapefile with 11 features (11 Italian cities) and I want them to become one, but I cannot activate the "Merge selected features" button. Any suggestion about the reason?

Mind that the project SR and the layer SR are the same and I already verified the topology. I attach a snapshot of the viewer and a link to download the shapefile.



[![Snapshot of the viewer][1]][1]
[Download the shapefile][2]


  [1]: https://i.sstatic.net/1JeRW.jpg
  [2]: https://dl.dropboxusercontent.com/u/23997272/comuni_merge.zip