diff --git a/additions/juggler/TargetRegistry.js b/additions/juggler/TargetRegistry.js
index 99665a1..32eaf45 100644
--- a/additions/juggler/TargetRegistry.js
+++ b/additions/juggler/TargetRegistry.js
@@ -383,11 +383,17 @@ class PageTarget {
     this._tab = tab;
     this._linkedBrowser = tab.linkedBrowser;
     this._browserContext = browserContext;
+    this._viewportSize = undefined;
     // Set the viewport size to Camoufox's default value.
-    this._viewportSize = {
-      width: ChromeUtils.camouGetInt("window.innerWidth") || 1280,
-      height: ChromeUtils.camouGetInt("window.innerHeight") || 720,
-    };;
+    if (
+      ChromeUtils.camouGetInt("window.innerWidth")
+      || ChromeUtils.camouGetInt("window.innerHeight")
+    ) {
+      this._viewportSize = {
+        width: ChromeUtils.camouGetInt("window.innerWidth") || 1280,
+        height: ChromeUtils.camouGetInt("window.innerHeight") || 720,
+      };
+    }
     this._initialDPPX = this._linkedBrowser.browsingContext.overrideDPPX;
     this._url = 'about:blank';
     this._openerId = opener ? opener.id() : undefined;