diff --git a/src/storage.rs b/src/storage.rs
index 8c63949..f0828ab 100644
--- a/src/storage.rs
+++ b/src/storage.rs
@@ -20,7 +20,7 @@ use itertools::Itertools;
 /// For completeness, the underlying, sorted scopes are provided as well. They might be
 /// useful for presentation to the user.
 pub trait TokenStorage {
-    type Error: 'static + Error + Send;
+    type Error: 'static + Error + Send + Sync;
 
     /// If `token` is None, it is invalid or revoked and should be removed from storage.
     /// Otherwise, it should be saved.
diff --git a/src/types.rs b/src/types.rs
index 72229b7..f44dc47 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -71,7 +71,7 @@ pub enum RequestError {
     /// An error occurred while refreshing tokens.
     Refresh(RefreshResult),
     /// Error in token cache layer
-    Cache(Box<dyn Error + Send>),
+    Cache(Box<dyn Error + Send + Sync>),
 }
 
 impl From<hyper::Error> for RequestError {