diff --git a/sdk/src/callback_signer.rs b/sdk/src/callback_signer.rs
index 519b5690..eccd4846 100644
--- a/sdk/src/callback_signer.rs
+++ b/sdk/src/callback_signer.rs
@@ -26,7 +26,7 @@ use crate::{
 /// The callback should return a signature for the given data.
 /// The callback should return an error if the data cannot be signed.
 pub type CallbackFunc =
-    dyn Fn(*const (), &[u8]) -> std::result::Result<Vec<u8>, Error>;
+    dyn Fn(*const (), &[u8]) -> std::result::Result<Vec<u8>, Error> + Send + Sync;
 
 /// Defines a signer that uses a callback to sign data.
 ///
@@ -59,7 +59,7 @@ impl CallbackSigner {
     /// Create a new callback signer.
     pub fn new<F, T>(callback: F, alg: SigningAlg, certs: T) -> Self
     where
-        F: Fn(*const (), &[u8]) -> std::result::Result<Vec<u8>, Error> + 'static,
+        F: Fn(*const (), &[u8]) -> std::result::Result<Vec<u8>, Error> + Send + Sync + 'static,
         T: Into<Vec<u8>>,
     {
         let certs = certs.into();