diff --git a/MLEM/Extensions/GraphicsExtensions.cs b/MLEM/Extensions/GraphicsExtensions.cs
index df5faa1..a84e01a 100644
--- a/MLEM/Extensions/GraphicsExtensions.cs
+++ b/MLEM/Extensions/GraphicsExtensions.cs
@@ -1,3 +1,4 @@
+using System;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
 
@@ -17,6 +18,9 @@ namespace MLEM.Extensions {
                 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;
             }