diff --git a/src/unix/bsd/freebsdlike/dragonfly/mod.rs b/src/unix/bsd/freebsdlike/dragonfly/mod.rs
index e739decb93b58..abe0440605752 100644
--- a/src/unix/bsd/freebsdlike/dragonfly/mod.rs
+++ b/src/unix/bsd/freebsdlike/dragonfly/mod.rs
@@ -1348,7 +1348,7 @@ pub const MINCORE_SUPER: ::c_int = 0x20;
 
 const_fn! {
     {const} fn _CMSG_ALIGN(n: usize) -> usize {
-        (n + ::mem::size_of::<::c_long>()) & !::mem::size_of::<::c_long>()
+        (n + (::mem::size_of::<::c_long>() - 1)) & !(::mem::size_of::<::c_long>() - 1)
     }
 }