diff --git a/src/Avalonia.X11/X11Window.cs b/src/Avalonia.X11/X11Window.cs
index 00761dfce8..5b9eebb9cb 100644
--- a/src/Avalonia.X11/X11Window.cs
+++ b/src/Avalonia.X11/X11Window.cs
@@ -100,14 +100,17 @@ namespace Avalonia.X11
 
             int defaultWidth = 300, defaultHeight = 200;
 
-            if (!_popup)
+            if (!_popup && Screen != null)
             {
                 var monitor = Screen.AllScreens.OrderBy(x => x.PixelDensity)
                    .FirstOrDefault(m => m.Bounds.Contains(Position));
 
-                // Emulate Window 7+'s default window size behavior.
-                defaultWidth = (int)(monitor.WorkingArea.Width * 0.75d);
-                defaultHeight = (int)(monitor.WorkingArea.Height * 0.7d);
+                if (monitor != null)
+                {
+                    // Emulate Window 7+'s default window size behavior.
+                    defaultWidth = (int)(monitor.WorkingArea.Width * 0.75d);
+                    defaultHeight = (int)(monitor.WorkingArea.Height * 0.7d);
+                }
             }
 
             _handle = XCreateWindow(_x11.Display, _x11.RootWindow, 10, 10, defaultWidth, defaultHeight, 0,