diff --git a/packages/happy-dom/src/window/Window.ts b/packages/happy-dom/src/window/Window.ts
index afa603e05..36690ca54 100644
--- a/packages/happy-dom/src/window/Window.ts
+++ b/packages/happy-dom/src/window/Window.ts
@@ -224,8 +224,8 @@ export default class Window extends EventTarget implements IWindow {
 			disableErrorCapturing: false,
 			enableFileSystemHttpRequests: false,
 			navigator: {
-				userAgent: `Mozilla/5.0 (X11; ${
-					process.platform.charAt(0).toUpperCase() + process.platform.slice(1) + ' ' + process.arch
+				userAgent: `Mozilla/5.0 (X11; ${(process?.platform) ?
+					process.platform.charAt(0).toUpperCase() + process.platform.slice(1) + ' ' + process.arch : 'Unknown'
 				}) AppleWebKit/537.36 (KHTML, like Gecko) HappyDOM/${PackageVersion.version}`
 			},
 			device: {