diff --git a/lib/operators.js b/lib/operators.js
index 6b92100..3abca76 100644
--- a/lib/operators.js
+++ b/lib/operators.js
@@ -53,7 +53,7 @@ export default {
   '!': {
     fn: (a) => {
       let sum = 1;
-      for (var i = 0; i < a; ++i) {
+      for (var i = 1; i <= a; ++i) {
         sum *= i;
       }
       return sum;