SQL Server 2012 – dtexec / val / proj my.ispac / pack my.dtsx / rep v will be returned immediately without any messages

I'm trying to set up a command call to dtexec to check packages using the project model. I call dtexec like this:

dtexec / val / proj my.ispac / pack my.dtsx / rep v 

The real paths to ispac and dtsx, however, are longer. When I do this, I only see on the console:

Microsoft (R) SQL Server utility for executing packages
Version 11.0.7001.0 for 32-bit
Copyright (C) 2016 Microsoft. All rights reserved.

Started at 10:40:01
PS D: >

The whole thing runs under a second, which makes me believe that it has done nothing. There are no messages, although I asked for detailed reporting.

Any ideas how that works?