diff --git a/src/sprites/rendered-target.js b/src/sprites/rendered-target.js
index 45cc3a87..23fc82dd 100644
--- a/src/sprites/rendered-target.js
+++ b/src/sprites/rendered-target.js
@@ -598,8 +598,9 @@ class RenderedTarget extends Target {
      * @return {number} Index of the named costume, or -1 if not present.
      */
     getCostumeIndexByName (costumeName) {
-        for (let i = 0; i < this.sprite.costumes.length; i++) {
-            if (this.getCostumes()[i].name === costumeName) {
+        const costumes = this.getCostumes();
+        for (let i = 0; i < costumes.length; i++) {
+            if (costumes[i].name === costumeName) {
                 return i;
             }
         }