diff --git a/Userland/Libraries/LibGUI/Window.cpp b/Userland/Libraries/LibGUI/Window.cpp
index 019737f006..c2b29d3e7e 100644
--- a/Userland/Libraries/LibGUI/Window.cpp
+++ b/Userland/Libraries/LibGUI/Window.cpp
@@ -948,6 +948,7 @@ bool Window::is_maximized() const
 
 void Window::set_maximized(bool maximized)
 {
+    VERIFY(m_window_id != 0);
     WindowServerConnection::the().async_set_maximized(m_window_id, maximized);
 }