diff --git a/src/MagicString.js b/src/MagicString.js
index b5b0ed2..2cccc0d 100644
--- a/src/MagicString.js
+++ b/src/MagicString.js
@@ -365,7 +365,7 @@ export default class MagicString {
 
 		if (storeName) {
 			const original = this.original.slice(start, end);
-			this.storedNames[original] = true;
+			Object.defineProperty(this.storedNames, original, { writable: true, value: true, enumerable: true });
 		}
 
 		const first = this.byStart[start];