How do Custom Exceptions work in Java | Junit

java.lang.AssertionError: 
Expected: (an instance of NotEnoughCoinsException and exception with message a 
string containing "Price 0.0")
but: exception with message a string containing "Price 0.0" message was null
Stacktrace was: NotEnoughCoinsException
at VendingMachine.selectProduct(VendingMachine.java:54)

    public String selectProduct(String productInput) throws NotEnoughCoinsException {
    switch (productInput) {
    case COLA:
        if(currentAmount >= ONE_DOLLAR)
            return COLA;
        else 
            throw new NotEnoughCoinsException("Price " + currentAmount);
    case CHIPS:
        return CHIPS;
    case CANDY:
        return CANDY;
    }
    return "";
}

   @Rule
public ExpectedException thrown = ExpectedException.none();

   @Test
public void shouldDisplayPriceWhenProductIsSelectedAndInsufficientAmountIsAvailable() throws  Exception {
    thrown.expect(NotEnoughCoinsException.class);
    thrown.expectMessage("Price " + vend.getCurrentAmount());
    vend.insertCoin(QUARTER);
    vend.selectProduct(COLA);
}

public class NotEnoughCoinsException extends Exception {

    private static final long serialVersionUID = 1L;

    public NotEnoughCoinsException(String message) {
    super();
    }
}