QUnit terminates when function with arg throws unless wrapped in anon function

I am using QUnit to test a function which throws an error in the case of invalid inputs. I am able to test this function by embedding it within an anonymous function, but am unsure why this is necessary. If I don't do this, QUnit reports:

Died on test #4     at http://localhost:8000/qunit/throwsTest.js:1:7: sometimesThrows

Notably, this message includes the thrown error.

I have also noticed that I can make functions that don't require args work if I pass the function as an argument rather than invoke it.

Check out my fiddle to see it live.

QUnit.test("throws test", function(assert) {
  function alwaysThrows() { throw "alwaysThrows"; }

  function sometimesThrows(foo) {
    if (foo) {
      return true;
    }
    else {
      throw "sometimesThrows";
    }
  }

  assert.throws(alwaysThrows, "alwaysThrows throws (works)")

  assert.ok(sometimesThrows(true), "sometimesThows doesn't throw (works)");
  assert.throws(function() {
    sometimesThrows(false);
  }, "sometimesThrows throws inside anonymous function (works)");

  // Where the unexpected behavior occurs.
  // The error _is_ being thrown being thrown.
  assert.throws(sometimesThrows(false), "sometimesThrows throws (terminates)");

});

If wrapping the function to be tested in an anonymous function is the intended method for getting this to work, if someone could explain why, I would find that illuminating in developing my intuition.