diff --git a/maths/digit_sum.ts b/maths/digit_sum.ts
index 34be6730..39724081 100644
--- a/maths/digit_sum.ts
+++ b/maths/digit_sum.ts
@@ -9,8 +9,12 @@
  */
 
 export const digitSum = (num: number): number => {
-  if (num < 0 || !Number.isInteger(num)) {
-    throw new Error('only natural numbers are supported')
+  if (!Number.isInteger(num)) {
+    throw new Error('only integers are supported')
+  }
+
+  if(num < 0) {
+    num *= -1;
   }
 
   let sum = 0