diff --git a/src/userdata/cell.rs b/src/userdata/cell.rs
index 5bd382f..40025fa 100644
--- a/src/userdata/cell.rs
+++ b/src/userdata/cell.rs
@@ -13,7 +13,7 @@ use super::r#ref::{UserDataRef, UserDataRefMut};
 type DynSerialize = dyn erased_serde::Serialize;
 
 #[cfg(all(feature = "serde", feature = "send"))]
-type DynSerialize = dyn erased_serde::Serialize + Send;
+type DynSerialize = dyn erased_serde::Serialize + Send + Sync;
 
 pub(crate) enum UserDataStorage<T> {
     Owned(UserDataVariant<T>),
diff --git a/src/userdata/lock.rs b/src/userdata/lock.rs
index e0e5d1a..b749ed7 100644
--- a/src/userdata/lock.rs
+++ b/src/userdata/lock.rs
@@ -72,7 +72,7 @@ mod lock_impl {
 
         #[inline(always)]
         fn try_lock_shared(&self) -> bool {
-            let flag = self.get().wrapping_add(1);
+            let flag = self.get().checked_add(1).expect("userdata lock count overflow");
             if flag <= UNUSED {
                 return false;
             }