diff --git a/src/opaque.rs b/src/opaque.rs
index e0f8ce2c2..2ca90af43 100644
--- a/src/opaque.rs
+++ b/src/opaque.rs
@@ -1,6 +1,7 @@
 #![allow(missing_docs)]
 
 use crate::void;
+use core::cell::UnsafeCell;
 use core::marker::{PhantomData, PhantomPinned};
 use core::mem;
 
@@ -14,7 +15,14 @@ use core::mem;
 pub struct Opaque {
     _private: [*const void; 0],
     _pinned: PhantomData<PhantomPinned>,
+    _mutable: SyncUnsafeCell<PhantomData<()>>,
 }
 
+// TODO: https://github.com/rust-lang/rust/issues/95439
+#[repr(transparent)]
+struct SyncUnsafeCell<T>(UnsafeCell<T>);
+
+unsafe impl<T> Sync for SyncUnsafeCell<T> {}
+
 const_assert_eq!(0, mem::size_of::<Opaque>());
 const_assert_eq!(1, mem::align_of::<Opaque>());