diff --git a/src/os/posix/x11.rs b/src/os/posix/x11.rs
index cf357b3..d79db05 100644
--- a/src/os/posix/x11.rs
+++ b/src/os/posix/x11.rs
@@ -724,7 +724,7 @@ impl Window {
                 y,
                 &mut nx,
                 &mut ny,
-                dummy_window.as_mut_ptr() as *mut u64,
+                dummy_window.as_mut_ptr() as *mut c_ulong,
             );
         }