diff --git a/src/lib.rs b/src/lib.rs
index f5b3d82..499b6c7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -190,6 +190,7 @@ impl fmt::Debug for LazyMutexId {
     }
 }
 
+/// Safety: the UnsafeCell is guaranteed to only be accessed mutably from a `Once`.
 unsafe impl Sync for LazyMutexId {}
 
 impl Deref for LazyMutexId {