WHEN_CALLED(...).Return(BY_VAL(s)) not working for std::string [closed]

std::string s{"one"};

WHEN_CALLED(returner->GetString()).Return(BY_VAL(s)); 

is throwing an exception.   Trying to follow the documentation from the "Behavior Settings Type" + "Return".

Here is my complete code (using catch2):

struct MyReturner

{

  int GetInt() { return -1; }

  std::string GetString() { return "-1"; }

};

TEST_CASE("mock return", "[mock]")

{

    MyReturner *returner = FAKE<MyReturner>();

    std::string s{"one"};

    WHEN_CALLED(returner->GetInt()).Return(1);

    WHEN_CALLED(returner->GetString()).Return(BY_VAL(s));

    REQUIRE( returner->GetInt() == 1);

    REQUIRE( returner->GetString() == "one" );

}    

Is there a unit test source examples posted somewhere that test most of the functionality of isolator++?