why does this code return -nan(ind)?c++

I'm trying to write a Scaler class that takes a child Test which has a Score.

The Scaler uses it's child's score class's scale method to rescale its value (ex. 1/1 with scale 5 = 5/5 or 0/1 with scale 5 = 0/5).

However whenever I run my code using the DummyTest class as a child it returns a "-nan(ind)/5 instead of 5/5 and my test case fails

TEST_CASE("Scaling test returning 1/1 to 5/5")
{
    auto test = std::make_shared<DummyTest>(Score(1, 1)); // Tests that always returns 1/1
    Scaler scaler(test, 5);

    CHECK(scaler.run() == Score(5, 5));
}

this is my code:

class Score {
public:
    double value;
    double maximum;

    Score() : value(0), maximum(0) { }                          
    Score(double maximum) : value(0), maximum(maximum) { }      
    Score(double value, double maximum) : value(value), maximum(maximum) { }

    Score scale(double scale) const;
    bool success();
};

Score Score::scale(double scale) const {
    double a = (value / maximum) * scale;
    double b = scale;
    return Score(a , b);
}

class Test {
public:
    virtual Score run() const;
};

class DummyTest : public Test {
    Score score;

public:
    DummyTest(const Score& score) : score(score) { }

    Score run() const override {
        return score;
    }
};

class Scaler : public Test {
public:
    Test child;
    double maximum;

    Scaler(std::shared_ptr<Test> child, double maximum) : child(*child), maximum(maximum) { }

    Score run() const override;

};

Score Scaler::run() const {
    Score c = child.run().scale(maximum);
    return c;
}