diff --git a/src/lib.rs b/src/lib.rs
index 38f5b39..e1bc79f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,5 +1,14 @@
 use reikna::totient::totient;
 
+fn gcd(mut a: i64, mut b: i64) -> i64 {
+    while b != 0 {
+        let temp = b;
+        b = a % b;
+        a = temp;
+    }
+    a.abs()
+}
+
 // Modular arithmetic functions using i64
 fn mod_add(a: i64, b: i64, p: i64) -> i64 {
     (a + b) % p
@@ -24,6 +33,7 @@ pub fn mod_exp(mut base: i64, mut exp: i64, p: i64) -> i64 {
 
 //compute the modular inverse of a modulo p using Fermat's little theorem, p not necessarily prime
 fn mod_inv(a: i64, p: i64) -> i64 {
+    assert!(gcd(a, p) == 1, "{} and {} are not coprime", a, p);
     mod_exp(a, totient(p as u64) as i64 - 1, p) // order of mult. group is Euler's totient function
 }
 
diff --git a/src/test.rs b/src/test.rs
index c0b0d4a..3553a79 100644
--- a/src/test.rs
+++ b/src/test.rs
@@ -50,7 +50,7 @@ mod tests {
 
     #[test]
     fn test_polymul_ntt_prime_power_modulus() {
-        let modulus: i64 = (17 as i64).pow(4); // modulus p^k or 2*p^k
+        let modulus: i64 = (17 as i64).pow(4); // modulus p^k
         let root: i64 = 3; // Primitive root of unity
         let n: usize = 8;  // Length of the NTT (must be a power of 2)
         let omega = omega(root, modulus, n); // n-th root of unity
@@ -70,4 +70,5 @@ mod tests {
         // Ensure both methods produce the same result
         assert_eq!(c_std, c_fast, "The results of polymul and polymul_ntt do not match");
     }
+    
 }