NUnit error: the test case source could not be found

This is quite simple but annoying behavior I am running into with NUnit:

I have one test like this:

   static string[] EvenStr = new string[] { "1", "2", "3", "4" };

   [Test, TestCaseSource("EvenStr")]
   public void TestMethod(string num)
   {
      Assert.IsTrue( 1 == 1 );
   }

When I run it I see this:

    One or more child tests had errors Exception doesn't have a stacktrace

    test case source could not be found

I use NUnit 3.10.1 and .net 4.5

Does anyone have any idea what is going on and why?