diff --git a/MLEM/Extensions/GraphicsExtensions.cs b/MLEM/Extensions/GraphicsExtensions.cs
index 4901c11..a84e01a 100644
--- a/MLEM/Extensions/GraphicsExtensions.cs
+++ b/MLEM/Extensions/GraphicsExtensions.cs
@@ -9,17 +9,18 @@ namespace MLEM.Extensions {
         private static int lastHeight;
 
         public static void SetFullscreen(this GraphicsDeviceManager manager, bool fullscreen) {
-            if (fullscreen || lastWidth == 0 || lastHeight == 0) {
-                var view = manager.GraphicsDevice.Viewport;
-                lastWidth = view.Width;
-                lastHeight = view.Height;
-            }
             manager.IsFullScreen = fullscreen;
             if (fullscreen) {
+                lastWidth = manager.GraphicsDevice.Viewport.Width;
+                lastHeight = manager.GraphicsDevice.Viewport.Height;
+
                 var curr = GraphicsAdapter.DefaultAdapter.CurrentDisplayMode;
                 manager.PreferredBackBufferWidth = curr.Width;
                 manager.PreferredBackBufferHeight = curr.Height;
             } else {
+                if (lastWidth <= 0 || lastHeight <= 0)
+                    throw new InvalidOperationException("Can't call SetFullscreen to change out of fullscreen mode without going into fullscreen mode first");
+
                 manager.PreferredBackBufferWidth = lastWidth;
                 manager.PreferredBackBufferHeight = lastHeight;
             }