@Test public void tag_mutableSpan_nullContext() {

  when(parseValue.apply(eq(input), isNull())).thenReturn("value");
  tag.tag(input, null, mutableSpan);
  verify(parseValue).apply(input, null);
  verifyNoMoreInteractions(parseValue); // doesn't parse twice
  MutableSpan expected = new MutableSpan();
  expected.tag("key", "value");
  assertThat(mutableSpan).isEqualTo(expected);
}