diff --git a/src/volume.ts b/src/volume.ts
index 28e625d5..f2e4bcc1 100644
--- a/src/volume.ts
+++ b/src/volume.ts
@@ -2131,7 +2131,7 @@ function emitStop(self) {
 }
 
 export class StatWatcher extends EventEmitter {
-  vol: Volume = null;
+  vol: Volume;
   filename: string;
   interval: number;
   timeoutRef?;