Testing a external dll for threadsafty with async unit test

Question

I'm trying to make a test for a external method to check if the method is thread safe. Multi user scenario. My code look like this.

    public async Task ComputeAirPressureDropThread_OK()
    {
        var tasks = new List<Task<double>>();

        for (int i = 1; i <= 10; i++)
        {
            Task<double> t = new Task<double>(() =>
                                              {
                                                  var friterm = new Coil();
                                                  return friterm.ComputeAirPressureDrop(150);
                                              }
                );

            t.Start();

            tasks.Add(t);
        }

        var results = await Task.WhenAll(tasks);

        foreach (var result in results)
        {
            Assert.AreEqual(result,
                            5.62194204005949,
                            0.0001);
        }
    }

The method ComputeAirPressureDrop(150) in the code above is what i tries to test if it is thread safe. But when running the test i get errors like this. The code is 4.6 and vs 2015. The function is a external COM dll with normal ref.