C Functions Compiler Error

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

bool isPrime(int test_num);

int main() {
    printf("Hello world!\n");
    if (isPrime(5)) {
        printf("5 is prime.");
    }
    return 0;
}

bool isPrime(int test_num) {
    /**
    Input:  test_num, an integer between two and MAXINT
    Output:  true if the number is a prime number (no other natural number divides
             it into an integer other than 1 and itself), false otherwise
    */
    bool prime = true;
    int i;

    if (test_num != 2)
    for(i = 3; i <= (test_num / 2); i++){
        if ((test_num % i) == 0) {
            prime = false;
            break;
        }
    }
    return prime;
}