diff --git a/src/entrypoints/tracing-headers.ts b/src/entrypoints/tracing-headers.ts
index 06abc4e01..a4148a45f 100644
--- a/src/entrypoints/tracing-headers.ts
+++ b/src/entrypoints/tracing-headers.ts
@@ -4,8 +4,12 @@ import { assignableWindow, window } from '../utils/globals'
 
 const addTracingHeaders = (sessionManager: SessionIdManager, req: Request) => {
     const { sessionId, windowId } = sessionManager.checkAndGetSessionAndWindowId(true)
-    req.headers.set('X-POSTHOG-SESSION-ID', sessionId)
-    req.headers.set('X-POSTHOG-WINDOW-ID', windowId)
+    if (sessionId) {
+        req.headers.set('X-POSTHOG-SESSION-ID', sessionId)
+    }
+    if (windowId) {
+        req.headers.set('X-POSTHOG-WINDOW-ID', windowId)
+    }
 }
 
 const patchFetch = (sessionManager: SessionIdManager): (() => void) => {
diff --git a/src/extensions/replay/sessionrecording.ts b/src/extensions/replay/sessionrecording.ts
index 030501aab..0200c6832 100644
--- a/src/extensions/replay/sessionrecording.ts
+++ b/src/extensions/replay/sessionrecording.ts
@@ -97,8 +97,8 @@ type SessionRecordingStatus = 'disabled' | 'sampled' | 'active' | 'buffering' |
 export interface SnapshotBuffer {
     size: number
     data: any[]
-    sessionId: string
-    windowId: string
+    sessionId: string | null
+    windowId: string | null
 }
 
 interface QueuedRRWebEvent {
@@ -252,8 +252,8 @@ export class SessionRecording {
 
     private _linkedFlagSeen: boolean = false
     private _lastActivityTimestamp: number = Date.now()
-    private windowId: string
-    private sessionId: string
+    private windowId: string | null
+    private sessionId: string | null
     private _linkedFlag: string | FlagVariant | null = null
 
     private _fullSnapshotTimer?: ReturnType<typeof setInterval>
@@ -315,6 +315,9 @@ export class SessionRecording {
     private get sessionDuration(): number | null {
         const mostRecentSnapshot = this.buffer?.data[this.buffer?.data.length - 1]
         const { sessionStartTimestamp } = this.sessionManager.checkAndGetSessionAndWindowId(true)
+        if (sessionStartTimestamp == null) {
+            return null
+        }
         return mostRecentSnapshot ? mostRecentSnapshot.timestamp - sessionStartTimestamp : null
     }
 
diff --git a/src/sessionid.ts b/src/sessionid.ts
index b2e31d759..0ef93b2b8 100644
--- a/src/sessionid.ts
+++ b/src/sessionid.ts
@@ -217,10 +217,10 @@ export class SessionIdManager {
     checkAndGetSessionAndWindowId(readOnly = false, _timestamp: number | null = null) {
         if (this.config.__preview_experimental_cookieless_mode) {
             return {
-                sessionId: undefined,
-                windowId: undefined,
-                sessionStartTimestamp: undefined,
-                lastActivityTimestamp: undefined,
+                sessionId: null,
+                windowId: null,
+                sessionStartTimestamp: null,
+                lastActivityTimestamp: null,
             }
         }
         const timestamp = _timestamp || new Date().getTime()