NUnit and testing a Xamarin PCL

I am developing a portable class library and iOS app where the data access is based on top of Paul Betts' ModernHttpClient, Fusillade, Refit and Akavache libraries. This works in the PCL and iOS apps but I am having an issue when creating unit tests against the PCL.

If I use the Resharper test running within Visual Studio 2015 all my tests pass but if I run the NUnit tests in Xamarin Studio about half the tests fail with TypeLoadExceptions. I have tried to determine which dll is causing me the problems but as far as I can see everything is as it should be.

PCL Class under test

   internal class ApiService<TRefitService> : IApiService<TRefitService> where TRefitService : IServiceContract
    {
        private readonly Lazy<TRefitService> background;
        private readonly Lazy<TRefitService> userInitiated;
        private readonly Lazy<TRefitService> speculative;

        private ApiService(){}

        public ApiService(string baseUrl)
        {
            ParameterGuard.ThrowIfNullOrEmpty(baseUrl, nameof(baseUrl));

            Func<string, RateLimitedHttpMessageHandler, TRefitService> createClient = (address, handler) =>
            {
                HttpClient client = new HttpClient(handler) {BaseAddress = new Uri(address)};
                return RestService.For<TRefitService>(client);
            };

            background = new Lazy<TRefitService>(() => createClient(baseUrl, new RateLimitedHttpMessageHandler(new NativeMessageHandler(), Priority.Background)));
            speculative = new Lazy<TRefitService>(() => createClient(baseUrl, new RateLimitedHttpMessageHandler(new NativeMessageHandler(), Priority.Speculative)));
            userInitiated = new Lazy<TRefitService>(() => createClient(baseUrl, new RateLimitedHttpMessageHandler(new NativeMessageHandler(), Priority.UserInitiated)));
        }

        /// <summary>
        /// Use for network requests that are running in the background.
        /// </summary>
        public TRefitService Background => background.Value; 

        /// <summary>
        /// Use to fetch data into a cache when a page loads. Expect that these requests will only get so far then give up and start failing.
        /// </summary>
        public TRefitService Speculative => speculative.Value;

        /// <summary>
        /// Use for network requests that are fetching data that the user is waiting on *right now*.
        /// </summary>
        public TRefitService UserInitiated => userInitiated.Value;
    }

Xamarin Studio Failing Test

        [Test()]
        public void PassingInANullBaseUrlThrowsAnException()
        {
            Assert.Throws<ArgumentNullException>(() => new ApiService<IAuthorisationApi>(null));
        }

Test Result

Expected: But was: (Could not load type 'ApiService`1' from assembly '/Users/206474978/Desktop/test/test/test/bin/Debug/xxx.xxx.xxx.dll'.) at NUnit.Framework.Assert.Throws (IResolveConstraint expression, NUnit.Framework.TestDelegate code, System.String message, System.Object[] args) <0x377c1c8 + 0x000e6> in :0

at NUnit.Framework.Assert.That (System.Object actual, IResolveConstraint expression, System.String message, System.Object[] args) <0x377c888 + 0x000bf> in :0 at NUnit.Framework.Assert.Throws (IResolveConstraint expression, NUnit.Framework.TestDelegate code, System.String message, System.Object[] args) <0x377c1c8 + 0x00137> in :0
at NUnit.Framework.Assert.Throws (System.Type expectedExceptionType, NUnit.Framework.TestDelegate code, System.String message, System.Object[] args) <0x377bfc8 + 0x0004f> in :0
at NUnit.Framework.Assert.Throws[T] (NUnit.Framework.TestDelegate code, System.String message, System.Object[] args) <0x377bf58 + 0x00033> in :0 at NUnit.Framework.Assert.Throws[T] (NUnit.Framework.TestDelegate code) <0x377bf08 + 0x0003b> in :0 at test.Test+ApiServiceTest.PassingInANullBaseUrlThrowsAnException () [0x00001] in /Users/206474978/Desktop/test/test/test/Test.cs:37 at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&) at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00038] in /private/tmp/source-mono-4.2.3/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.3/mcs/class/corlib/System.Reflection/MonoMethod.cs:295

Now this wouldn't normally be an issue as I would just use reshaper but I am building our Continuous Integration pipeline on top of Jenkins and running the tests via the nunit package within the Xamarin libraries which results in the build reporting failures.

Any idea what the problem is here?