diff --git a/Source/OpenTK/Platform/Windows/API.cs b/Source/OpenTK/Platform/Windows/API.cs
index a52e1587..42701067 100644
--- a/Source/OpenTK/Platform/Windows/API.cs
+++ b/Source/OpenTK/Platform/Windows/API.cs
@@ -2445,6 +2445,7 @@ namespace OpenTK.Platform.Windows
 
     #region internal enum WindowStyle : int
 
+    [Flags]
     internal enum WindowStyle : int
     {
         Overlapped = 0x00000000,