diff --git a/src/logging/IDBLogger.ts b/src/logging/IDBLogger.ts
index d5ce2f9a..dcfd3a64 100644
--- a/src/logging/IDBLogger.ts
+++ b/src/logging/IDBLogger.ts
@@ -118,7 +118,7 @@ export class IDBLogger extends BaseLogger {
     }
     
     _persistItem(logItem: ILogItem, filter: LogFilter, forced: boolean): void {
-        const serializedItem = logItem.serialize(filter, null, forced);
+        const serializedItem = logItem.serialize(filter, undefined, forced);
         this._queuedItems.push({
             json: JSON.stringify(serializedItem)
         });
diff --git a/src/logging/LogItem.ts b/src/logging/LogItem.ts
index 012abc57..82f785e4 100644
--- a/src/logging/LogItem.ts
+++ b/src/logging/LogItem.ts
@@ -53,7 +53,7 @@ export interface ILogItem {
     catch(err: Error): Error;
     finish(): void;
     child(labelOrValues: LabelOrValues, logLevel?: LogLevel, filterCreator?: FilterCreator): ILogItem;
-    serialize(filter: LogFilter, parentStartTime: number | null, forced: boolean): ISerializedItem | null;
+    serialize(filter: LogFilter, parentStartTime: number | undefined, forced: boolean): ISerializedItem | undefined;
 }
 
 export type LogItemValues = {
@@ -171,7 +171,7 @@ export class LogItem implements ILogItem {
     }
 
     // todo: null or undefined here?
-    serialize(filter: LogFilter, parentStartTime: number | null = null, forced: boolean): ISerializedItem | null {
+    serialize(filter: LogFilter, parentStartTime: number | undefined, forced: boolean): ISerializedItem | undefined {
         if (this._filterCreator) {
             try {
                 filter = this._filterCreator(new LogFilter(filter), this);
@@ -193,12 +193,12 @@ export class LogItem implements ILogItem {
             }, null);
         }
         if (filter && !filter.filter(this, children)) {
-            return null;
+            return;
         }
         // in (v)alues, (l)abel and (t)ype are also reserved.
         const item: ISerializedItem = {
             // (s)tart
-            s: parentStartTime === null ? this.start : this.start - parentStartTime,
+            s: parentStartTime? this.start - parentStartTime : this.start,
             // (d)uration
             d: this.duration,
             // (v)alues
diff --git a/src/logging/NullLogger.ts b/src/logging/NullLogger.ts
index 601a1c39..63e91f61 100644
--- a/src/logging/NullLogger.ts
+++ b/src/logging/NullLogger.ts
@@ -102,7 +102,7 @@ export class NullLogItem implements ILogItem {
     finish(): void {}
 
     serialize() {
-        return null;
+        return undefined;
     }
 }