diff --git a/Cargo.toml b/Cargo.toml
index a7d4a78..529c35b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "stretto"
-version = "0.4.4"
+version = "0.4.6"
 authors = ["Al Liu <scygliu1@gmail.com>"]
 description = "Stretto is a high performance thread-safe memory-bound Rust cache."
 homepage = "https://github.com/al8n/stretto"
@@ -47,14 +47,14 @@ parking_lot = "0.12"
 rand = "0.8"
 serde = {version = "1", optional = true}
 serde_json = {version = "1", optional = true}
-tokio = { version = "1.17", optional = true }
+tokio = { version = "1.19", optional = true }
 twox-hash = "1.6"
 wg = "0.2"
 
 [dev-dependencies]
 serde = {version = "1", features = ["serde_derive"]}
 serde_json = "1"
-tokio = {version = "1.17", features = ["rt-multi-thread", "test-util"]}
+tokio = {version = "1.19", features = ["rt-multi-thread", "test-util"]}
 
 [package.metadata.docs.rs]
 all-features = true
diff --git a/src/cache/axync.rs b/src/cache/axync.rs
index 20e3948..cf0977c 100644
--- a/src/cache/axync.rs
+++ b/src/cache/axync.rs
@@ -390,7 +390,7 @@ impl<K, V, KH, C, U, CB, S> AsyncCache<K, V, KH, C, U, CB, S>
         C: Coster<V>,
         U: UpdateValidator<V>,
         CB: CacheCallback<V>,
-        S: BuildHasher + Clone + 'static,
+        S: BuildHasher + Clone + 'static + Send,
 {
     /// `insert` attempts to add the key-value item to the cache. If it returns false,
     /// then the `insert` was dropped and the key-value item isn't added to the cache. If
diff --git a/src/cache/test.rs b/src/cache/test.rs
index c4cbba3..849a246 100644
--- a/src/cache/test.rs
+++ b/src/cache/test.rs
@@ -696,15 +696,14 @@ mod sync_test {
     fn test_valueref_ttl() {
         let ttl = Duration::from_secs(1);
         let c = Cache::builder(100, 1000)
-        .set_key_builder(TransparentKeyBuilder::default())
-        .set_metrics(true)
-        .finalize()
-        .unwrap();
+            .set_key_builder(TransparentKeyBuilder::default())
+            .set_metrics(true)
+            .finalize()
+            .unwrap();
         c.try_insert_with_ttl(1, 1, 1, ttl).unwrap();
         c.wait().unwrap();
         let val = c.get(&1).unwrap();
-        assert!(val.ttl()>Duration::from_millis(900));
-
+        assert!(val.ttl() > Duration::from_millis(900));
     }
 }
 
diff --git a/src/policy.rs b/src/policy.rs
index ff0bc02..1bb8d52 100644
--- a/src/policy.rs
+++ b/src/policy.rs
@@ -23,8 +23,8 @@ const DEFAULT_SAMPLES: usize = 5;
 
 macro_rules! impl_policy {
     ($policy: ident) => {
-        use crate::policy::DEFAULT_SAMPLES;
         use crate::policy::PolicyPair;
+        use crate::policy::DEFAULT_SAMPLES;
 
         impl $policy {
             #[inline]
diff --git a/src/policy/axync.rs b/src/policy/axync.rs
index 057ba86..58f430d 100644
--- a/src/policy/axync.rs
+++ b/src/policy/axync.rs
@@ -15,7 +15,7 @@ pub(crate) struct AsyncLFUPolicy<S = RandomState> {
     pub(crate) metrics: Arc<Metrics>,
 }
 
-impl<S: BuildHasher + Clone + 'static> AsyncLFUPolicy<S> {
+impl<S: BuildHasher + Clone + 'static + Send> AsyncLFUPolicy<S> {
     #[inline]
     pub fn with_hasher(ctrs: usize, max_cost: i64, hasher: S) -> Result<Self, CacheError> {
         let inner = PolicyInner::with_hasher(ctrs, max_cost, hasher)?;
@@ -84,7 +84,7 @@ pub(crate) struct PolicyProcessor<S: BuildHasher + Clone + 'static> {
     stop_rx: Receiver<()>,
 }
 
-impl<S: BuildHasher + Clone + 'static> PolicyProcessor<S> {
+impl<S: BuildHasher + Clone + 'static + Send> PolicyProcessor<S> {
     #[inline]
     fn new(
         inner: Arc<Mutex<PolicyInner<S>>>,
@@ -130,8 +130,8 @@ impl<S: BuildHasher + Clone + 'static> PolicyProcessor<S> {
 
 }
 
-unsafe impl<S: BuildHasher + Clone + 'static> Send for PolicyProcessor<S> {}
-unsafe impl<S: BuildHasher + Clone + 'static> Sync for PolicyProcessor<S> {}
+unsafe impl<S: BuildHasher + Clone + 'static + Send> Send for PolicyProcessor<S> {}
+unsafe impl<S: BuildHasher + Clone + 'static + Send + Sync> Sync for PolicyProcessor<S> {}
 
 
 impl_policy!(AsyncLFUPolicy);
\ No newline at end of file