diff --git a/src/core/lib/Ciphers.mjs b/src/core/lib/Ciphers.mjs
index a4d174b5..6266a8e1 100644
--- a/src/core/lib/Ciphers.mjs
+++ b/src/core/lib/Ciphers.mjs
@@ -3,6 +3,7 @@
  *
  * @author Matt C [matt@artemisbot.uk]
  * @author n1474335 [n1474335@gmail.com]
+ * @author Evie H [evie@evie.sh]
  *
  * @copyright Crown Copyright 2018
  * @license Apache-2.0
@@ -10,6 +11,7 @@
  */
 
 import OperationError from "../errors/OperationError.mjs";
+import Utils from "../Utils.mjs";
 import CryptoJS from "crypto-js";
 
 /**
@@ -30,6 +32,10 @@ export function affineEncode(input, args) {
         throw new OperationError("The values of a and b can only be integers.");
     }
 
+    if (Utils.gcd(a, 26) !== 1) {
+        throw new OperationError("The value of `a` must be coprime to 26.");
+    }
+
     for (let i = 0; i < input.length; i++) {
         if (alphabet.indexOf(input[i]) >= 0) {
             // Uses the affine function ax+b % m = y (where m is length of the alphabet)