F# & visual studio : I am unable to run tests

I try to learn F# with visual studio and I created a solution with 2 projects : FSharpCalled (which contains a method "disBonjour", transforming a string in another), an F# library, and FSharpTest shown below :

here's FSharpCalled :

the problem is I can't run the test; as shwon in the first screenshot, I get an error message saying the "principal module is empty, so nothing will happen at execution".

I tried to modify some properties but without success.

thanks,

olivier