diff --git a/webapp/frontend/src/app/shared/device-hours.pipe.spec.ts b/webapp/frontend/src/app/shared/device-hours.pipe.spec.ts
index 86b3aa6..cabad90 100644
--- a/webapp/frontend/src/app/shared/device-hours.pipe.spec.ts
+++ b/webapp/frontend/src/app/shared/device-hours.pipe.spec.ts
@@ -31,19 +31,17 @@ describe("DeviceHoursPipe", () => {
       {
         input: null,
         configuration: "device_hours",
-        result: "null hours",
+        result: "Unknown",
       },
       {
         input: null,
         configuration: "humanize",
-        result: "0 seconds",
+        result: "Unknown",
       },
     ];
 
     testCases.forEach((test, index) => {
-      it(`should format input ${test.input} with configuration '${
-        test.configuration
-      }' (testcase: ${index + 1})`, () => {
+      it(`format input '${test.input}' with configuration '${test.configuration}', should be '${test.result}' (testcase: ${index + 1})`, () => {
         // test
         const pipe = new DeviceHoursPipe();
         const formatted = pipe.transform(test.input, test.configuration);
diff --git a/webapp/frontend/src/app/shared/device-hours.pipe.ts b/webapp/frontend/src/app/shared/device-hours.pipe.ts
index f6b5e1f..1170c8a 100644
--- a/webapp/frontend/src/app/shared/device-hours.pipe.ts
+++ b/webapp/frontend/src/app/shared/device-hours.pipe.ts
@@ -4,7 +4,10 @@ import humanizeDuration from 'humanize-duration';
 @Pipe({ name: 'deviceHours' })
 export class DeviceHoursPipe implements PipeTransform {
     static format(hoursOfRunTime: number, unit: string, humanizeConfig: object): string {
-        if (unit === 'device_hours') {
+      if (hoursOfRunTime === null) {
+        return 'Unknown';
+      }
+      if (unit === 'device_hours') {
           return `${hoursOfRunTime} hours`;
         }
         return humanizeDuration(hoursOfRunTime * 60 * 60 * 1000, humanizeConfig);