diff --git a/src/FieldsArray.ts b/src/FieldsArray.ts
index ab08494..f0aa40e 100644
--- a/src/FieldsArray.ts
+++ b/src/FieldsArray.ts
@@ -1,4 +1,7 @@
+import PKPass from "./PKPass";
+import { fieldKeysPoolSymbol } from "./PKPass";
 import * as Schemas from "./schemas";
+import * as Utils from "./utils";
 import formatMessage, * as Messages from "./messages";
 
 /**
@@ -6,14 +9,17 @@ import formatMessage, * as Messages from "./messages";
  * @see https://apple.co/2wkUBdh
  */
 
-const poolSymbol = Symbol("pool");
+const passInstanceSymbol = Symbol("passInstance");
 
 export default class FieldsArray extends Array<Schemas.Field> {
-	private [poolSymbol]: Set<string>;
+	private [passInstanceSymbol]: InstanceType<typeof PKPass>;
 
-	constructor(pool: Set<string>, ...args: Schemas.Field[]) {
+	constructor(
+		passInstance: InstanceType<typeof PKPass>,
+		...args: Schemas.Field[]
+	) {
 		super(...args);
-		this[poolSymbol] = pool;
+		this[passInstanceSymbol] = passInstance;
 	}
 
 	/**
@@ -22,6 +28,8 @@ export default class FieldsArray extends Array<Schemas.Field> {
 	 */
 
 	push(...fieldsData: Schemas.Field[]): number {
+		Utils.assertUnfrozen(this[passInstanceSymbol]);
+
 		const validFields = fieldsData.reduce(
 			(acc: Schemas.Field[], current: Schemas.Field) => {
 				try {
@@ -35,7 +43,9 @@ export default class FieldsArray extends Array<Schemas.Field> {
 					return acc;
 				}
 
-				if (this[poolSymbol].has(current.key)) {
+				const pool = this[passInstanceSymbol][fieldKeysPoolSymbol];
+
+				if (pool.has(current.key)) {
 					console.warn(
 						formatMessage(
 							Messages.FIELDS.REPEATED_KEY,
@@ -45,7 +55,7 @@ export default class FieldsArray extends Array<Schemas.Field> {
 					return acc;
 				}
 
-				this[poolSymbol].add(current.key);
+				pool.add(current.key);
 				return [...acc, current];
 			},
 			[],
@@ -60,8 +70,10 @@ export default class FieldsArray extends Array<Schemas.Field> {
 	 */
 
 	pop(): Schemas.Field {
+		Utils.assertUnfrozen(this[passInstanceSymbol]);
+
 		const element: Schemas.Field = super.pop();
-		this[poolSymbol].delete(element.key);
+		this[passInstanceSymbol][fieldKeysPoolSymbol].delete(element.key);
 		return element;
 	}
 
@@ -75,8 +87,12 @@ export default class FieldsArray extends Array<Schemas.Field> {
 		deleteCount: number,
 		...items: Schemas.Field[]
 	): Schemas.Field[] {
+		Utils.assertUnfrozen(this[passInstanceSymbol]);
+
 		const removeList = this.slice(start, deleteCount + start);
-		removeList.forEach((item) => this[poolSymbol].delete(item.key));
+		removeList.forEach((item) =>
+			this[passInstanceSymbol][fieldKeysPoolSymbol].delete(item.key),
+		);
 
 		return super.splice(start, deleteCount, ...items);
 	}
diff --git a/src/PKPass.ts b/src/PKPass.ts
index 081ae9b..c0e464f 100644
--- a/src/PKPass.ts
+++ b/src/PKPass.ts
@@ -305,11 +305,11 @@ export default class PKPass extends Bundle {
 
 		this[passTypeSymbol] = type;
 		this[propsSymbol][this[passTypeSymbol]] = {
-			headerFields /******/: new FieldsArray(this[fieldKeysPoolSymbol]),
-			primaryFields /*****/: new FieldsArray(this[fieldKeysPoolSymbol]),
-			secondaryFields /***/: new FieldsArray(this[fieldKeysPoolSymbol]),
-			auxiliaryFields /***/: new FieldsArray(this[fieldKeysPoolSymbol]),
-			backFields /********/: new FieldsArray(this[fieldKeysPoolSymbol]),
+			headerFields /******/: new FieldsArray(this),
+			primaryFields /*****/: new FieldsArray(this),
+			secondaryFields /***/: new FieldsArray(this),
+			auxiliaryFields /***/: new FieldsArray(this),
+			backFields /********/: new FieldsArray(this),
 			transitType: undefined,
 		};
 	}
diff --git a/src/utils.ts b/src/utils.ts
index 6636a9d..d15e9d3 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,3 +1,6 @@
+import * as Messages from "./messages";
+import type Bundle from "./Bundle";
+
 /**
  * Acts as a wrapper for converting date to W3C string
  * @param date
@@ -105,3 +108,9 @@ export function cloneRecursive(object: Object) {
 
 	return objectCopy;
 }
+
+export function assertUnfrozen(instance: InstanceType<typeof Bundle>) {
+	if (instance.isFrozen) {
+		throw new Error(Messages.BUNDLE.CLOSED);
+	}
+}