FitNesse Error : Could not invoke constructor (FitNesse slim)

I am getting an error "Could not invoke constructor" after I tried to test the code through FitNesse. What can I do to solve this problem?

I have followed instructions from "https://www.softwaretestinghelp.com/getting-started-with-fitnesse-a-collaboration-tool-for-testers-and-developers/" website.

This is the code I wrote to test:

public class Calculator {

public int first, second;

public void setFirst(int first) {
    this.first = first;
}

public void setSecond(int second) {
    this.second = second;
}

public int addition() {
    return (first+second);
}

}

This is what I wrote in FitNesse :

!define TEST_SYSTEM {slim} !path F:\Eclipse\Workspace\TestFitNesse\bin

| Calculator |

|first |second|addition?|

|4 |2 |6 |

Below is the result I get when I click on the 'Test' button:

Link to view the screenshot of the error