I`m working on a model for 3d printing. I wanted to make it hollow, so i made a duplicate with Shift+D and wanted to scale it inwards with Alt+S... I expected something like in this question explained: Alt+S and S scaling
scaling with S I get something like that

but with Alt+S I get this and cannot scale :(

scaling only with S I don't get an evenly thick frame... so I don't know how to solve this.
Thank you in advance and happy modeling :)