How do I test custom error messages with the help of CUnit?

Hello everyone I am new to Cunit.My question is below the code so you may skip it if you want to. I made a small function that adds two positive integers and returns the result and if any of the integers is less than or equal to zero then it writes to stderr an error message.

/*This file is dummy.c*/
#include "dummy.h"

int add(int a,int b)
{
   if(a<=0||b<=0)
   {
       fprintf(stderr,"Error in arguments");
       exit(EXIT_FAILURE);
   }
   else
       return a+b;
}

This is dummy.h

#include<stdio.h>
#include<stdlib.h>

int add(int a,int b)

This is a part of my CUnit test code. Filename is testdummy.c

/*Necessary files have been included*/
void addtest()
{
    CU_ASSERT_EQUAL(add(3,5),8);
    CU_ASSERT_EQUAL(add(1,6),7);
    /*What should I add here*/
}

What should I add in the section specified so that my code is also tested for error messages?