WebApi controller tests method not found

I have a very simple WebAPI 2 controller running on .NET Framework 4.6.2, that looks like this:

[RoutePrefix("Invitations")]
public class InvitationsController : CqrsApiController
{
    [HttpPost, Route("Clients/{id:long}/Actions/Cancel")]
    public IHttpActionResult PostClientInvitationCancel(long id, [FromBody] ClientInvitationCancelCommand command)
    {
        Execute(command);
        return SeeOther("Invitations/Clients/{0}", id);
    }
}

and am trying to write an NUnit test for it, like this:

[TestFixture]
public class WhenExecutingAValidCommand
{
    [Test]
    public void ItShouldReturnARedirect()
    {
        var dispatcher = Substitute.For<ICqrsDispatcher>();
        var urlHelper = Substitute.For<UrlHelper>();
        urlHelper.Link(Arg.Any<string>(), Arg.Any<object>()).Returns("https://tempuri.org/");

        var sut = new InvitationsController(dispatcher);
        sut.Request = new HttpRequestMessage();
        sut.Configuration = new HttpConfiguration();
        sut.Url = urlHelper;

        var response = sut.PostClientInvitationCancel(1, new ClientInvitationCancelCommand());
        response.Should().BeOfType<SeeOtherRedirectResult>();
    }
}

```

However, when I run the test, I get the following error:

System.MissingMethodException : Method not found: 'Void System.Web.Http.ApiController.set_Request(System.Net.Http.HttpRequestMessage)'.
   at ApiProjectTests.InvitationsControllerTests.WhenExecutingAValidCommand.ItShouldReturnARedirect()

The same code seems to work fine in similar projects based on .NET Framework 4.5.1, so I'm wondering if there's some sort of DLL hell going on here. System.Web.Http is using Microsoft.AspNet.WebApi.Core.5.2.3, whereas System.Net.Http is coming from the GAC (well, C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\Microsoft\Microsoft.NET.Build.Extensions\net461\lib\System.Net.Http.dll to be more precise).