diff --git a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
index cbfa1abfb7..5c364c4038 100644
--- a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
+++ b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
@@ -460,6 +460,7 @@ namespace Avalonia.Win32.Interop
             WS_SIZEFRAME = 0x40000,
             WS_SYSMENU = 0x80000,
             WS_TABSTOP = 0x10000,
+            WS_THICKFRAME = 0x40000,
             WS_VISIBLE = 0x10000000,
             WS_VSCROLL = 0x200000,
             WS_EX_DLGMODALFRAME = 0x00000001,