diff --git a/ts/early.hrtMeasurement.ts b/ts/early.hrtMeasurement.ts
index 190f4db..1ca426e 100644
--- a/ts/early.hrtMeasurement.ts
+++ b/ts/early.hrtMeasurement.ts
@@ -6,8 +6,8 @@ import * as process from 'process';
 export class HrtMeasurement {
   nanoSeconds: number = null;
   milliSeconds: number = null;
-  private _hrTimeStart = null;
-  private _hrTimeStopDiff = null;
+  private _hrTimeStart: [number, number] = null;
+  private _hrTimeStopDiff: [number, number] = null;
   private _started: boolean = false;
 
   /**