diff --git a/src/templates/core/CancelablePromise.hbs b/src/templates/core/CancelablePromise.hbs
index 09765790..b708b4ed 100644
--- a/src/templates/core/CancelablePromise.hbs
+++ b/src/templates/core/CancelablePromise.hbs
@@ -21,7 +21,7 @@ export interface OnCancel {
 }
 
 export class CancelablePromise<T> implements Promise<T> {
-	readonly [Symbol.toStringTag]: string;
+	readonly [Symbol.toStringTag]!: string;
 
 	private _isResolved: boolean;
 	private _isRejected: boolean;