diff --git a/src/plugin/duration/index.js b/src/plugin/duration/index.js
index e2060c344..7c35c01d0 100644
--- a/src/plugin/duration/index.js
+++ b/src/plugin/duration/index.js
@@ -61,6 +61,11 @@ class Duration {
   constructor(input, unit, locale) {
     this.$d = {}
     this.$l = locale
+
+    Object.defineProperty(this, 'valueOf', {
+      value: () => this.asMilliseconds()
+    })
+
     if (input === undefined) {
       this.$ms = 0
       this.parseFromMilliseconds()
@@ -236,10 +241,6 @@ class Duration {
       .fromNow(!withSuffix)
   }
 
-  valueOf() {
-    return this.asMilliseconds()
-  }
-
   milliseconds() { return this.get('milliseconds') }
   asMilliseconds() { return this.as('milliseconds') }
   seconds() { return this.get('seconds') }