Calling Another Function in a Recusive Function results in error. Why?

I've two functions fun() and fun2();

fun() calls itself while incrementing the global variable a=0 and terminates if a==5. But if I call another function called fun2() which basically returns and do nothing, then Build Error happens. Why? I'm just experimenting with recursion and I got this error.

#include<iostream>
using namespace std;
int a = 0;

void fun() {
    if (a == 5)
        return;
    a++;
    fun();
    fun2();  //This is where the trouble happens
}
        
void fun2() {
    return;
}

int main() {
    fun();
    return 0;
}

Output: