diff --git a/packages/astro/src/transitions/events.ts b/packages/astro/src/transitions/events.ts
index 969a121399..a35fd72cb6 100644
--- a/packages/astro/src/transitions/events.ts
+++ b/packages/astro/src/transitions/events.ts
@@ -25,7 +25,7 @@ class BeforeEvent extends Event {
 	readonly sourceElement: Element | undefined;
 	readonly info: any;
 	newDocument: Document;
-	signal: AbortSignal;
+	readonly signal: AbortSignal;
 
 	constructor(
 		type: string,