diff --git a/src/volume.ts b/src/volume.ts
index ed7b2ce6..b14c294b 100644
--- a/src/volume.ts
+++ b/src/volume.ts
@@ -2129,7 +2129,7 @@ export class StatWatcher extends EventEmitter {
   vol: Volume = null;
   filename: string;
   interval: number;
-  timeoutRef = null;
+  timeoutRef?;
   setTimeout: TSetTimeout;
   prev: Stats = null;