diff --git a/docx-wasm/js/abstract-numbering.ts b/docx-wasm/js/abstract-numbering.ts
index 3db5f35..1cc6856 100644
--- a/docx-wasm/js/abstract-numbering.ts
+++ b/docx-wasm/js/abstract-numbering.ts
@@ -2,7 +2,7 @@ import { Level } from "./level";
 
 export class AbstractNumbering {
   id: number;
-  levels: Level[];
+  levels: Level[] = [];
   constructor(id: number) {
     this.id = id;
   }