diff --git a/src/plugin/weekOfYear/index.js b/src/plugin/weekOfYear/index.js
index b7c301361..e2fc9b3e8 100644
--- a/src/plugin/weekOfYear/index.js
+++ b/src/plugin/weekOfYear/index.js
@@ -6,11 +6,12 @@ export default (o, c, d) => {
     if (week !== null) {
       return this.add((week - this.week()) * 7, 'day')
     }
-    const endOfYear = this.endOf(Y)
+    // d(this) clone is for badMutable plugin
+    const endOfYear = d(this).endOf(Y)
     if (endOfYear.day() !== 6 && this.month() === 11 && (31 - this.date()) <= endOfYear.day()) {
       return 1
     }
-    const startOfYear = d(this.$d).startOf(Y)
+    const startOfYear = d(this).startOf(Y)
     const compareDay = startOfYear.subtract(startOfYear.day(), D).subtract(1, MS)
     const diffInWeek = this.diff(compareDay, W, true)
     return Math.ceil(diffInWeek)