diff --git a/src/index.js b/src/index.js
index 58bb30423..bc036c48f 100644
--- a/src/index.js
+++ b/src/index.js
@@ -422,7 +422,7 @@ class Dayjs {
     // ie 8 return
     // new Dayjs(this.valueOf() + this.$d.getTimezoneOffset() * 60000)
     // .format('YYYY-MM-DDTHH:mm:ss.SSS[Z]')
-    return this.toDate().toISOString()
+    return this.$d.toISOString()
   }
 
   toObject() {