NUnit fails on F# project with FsCheck

[sideeffffect]

I have a trivial project started from ProjectScaffold.
I have a test file tests/FSharpHello.Tests/Tests.fs like this:

module FSharpHello.Tests

open FSharpHello
open NUnit.Framework
open FsUnit
open FsCheck.NUnit

[<Test>]
let ``hello returns 42`` () =
  Library.hello 42 |> should equal 42

[<Property>]
let ``hello always returns 42``(x:int) =
  Library.hello x = 42

on ./build.sh I get this error:

mono  /home/ondra/Projects/FSharpHello/packages/NUnit.Runners/tools/nunit-console.exe "-nologo" "-noshadow" "-labels" "/home/ondra/Projects/FSharpHello/tests/FSharpHello.Tests/bin/Release/FSharpHello.Tests.dll" "-xml:TestResults.xml" 
WARNING: The runtime version supported by this application is unavailable.
Using default runtime: v4.0.30319
ProcessModel: Default    DomainUsage: Single
Execution Runtime: mono-4.0
***** FSharpHello.Tests.hello always returns 42
***** FSharpHello.Tests.hello returns 42

Tests run: 1, Errors: 0, Failures: 0, Inconclusive: 0, Time: 0.0393985 seconds
  Not run: 1, Invalid: 1, Ignored: 0, Skipped: 0

Errors and Failures:

Tests Not Run:
1) NotRunnable : FSharpHello.Tests.hello always returns 42
   Test method has non-void return type, but no result is expected

Running build failed.
Error:
Fake.UnitTestCommon+FailedTestsException: NUnit test failed (1).
  at Fake.NUnitSequential.NUnit (Microsoft.FSharp.Core.FSharpFunc`2 setParams, IEnumerable`1 assemblies) [0x00000] in <filename unknown>:0 
  at FSI_0001.Build+clo@137-11.Invoke (Microsoft.FSharp.Core.Unit _arg7) [0x00000] in <filename unknown>:0 
  at Fake.TargetHelper+targetFromTemplate@155[Microsoft.FSharp.Core.Unit].Invoke (Microsoft.FSharp.Core.Unit unitVar0) [0x00000] in <filename unknown>:0 
  at Fake.TargetHelper.runSingleTarget (Fake.TargetTemplate`1 target) [0x00000] in <filename unknown>:0 

when build or run tests in MonoDevelop, hello returns 42 runs OK, but hello always returns 42 doesn't run because of this error

Test method has non-void return type, but no result is expected

without hello always returns 42 everything is ok.

I've also reported this problem at FAKE.

---

link to the project and failing travis build

[rprouse]

I am closing this because it has been inactive for so long. If anyone has further info, we can reopen.