public FEELFnResult<String> invoke(@ParameterName("string") String string) {
        if ( string == null ) {
            return FEELFnResult.ofError(new InvalidParametersEvent(Severity.ERROR, "string", "cannot be null"));
        } else {
            return FEELFnResult.ofResult( string.toUpperCase() );
        }
    }
@Test
    void invokeNull() {
        FunctionTestUtil.assertResultError(stringUpperCaseFunction.invoke(null), InvalidParametersEvent.class);
    }