@Test public void injectorFunction_null() {

  InjectorFunction existing = mock(InjectorFunction.class);
  assertThatThrownBy(() -> injectorFunction(existing, null))
      .hasMessage("injectorFunctions == null");
  assertThatThrownBy(() -> injectorFunction(existing, new InjectorFunction[] {null}))
      .hasMessage("injectorFunction == null");
  assertThatThrownBy(() -> injectorFunction(existing, one, null))
      .hasMessage("injectorFunction == null");
}