diff --git a/dlls/win32u/ntuser_private.h b/dlls/win32u/ntuser_private.h
index 4e13822c6fb..385e7c0ded5 100644
--- a/dlls/win32u/ntuser_private.h
+++ b/dlls/win32u/ntuser_private.h
@@ -253,7 +253,7 @@ static inline UINT win_get_flags( HWND hwnd )
 }
 
 WND *get_win_ptr( HWND hwnd ) DECLSPEC_HIDDEN;
-BOOL is_child( HWND parent, HWND child );
+BOOL is_child( HWND parent, HWND child ) DECLSPEC_HIDDEN;
 BOOL is_window( HWND hwnd ) DECLSPEC_HIDDEN;
 
 #endif /* __WINE_NTUSER_PRIVATE_H */