diff --git a/crates/oidc-client/src/types/client_credentials.rs b/crates/oidc-client/src/types/client_credentials.rs
index 3d057621..45d63005 100644
--- a/crates/oidc-client/src/types/client_credentials.rs
+++ b/crates/oidc-client/src/types/client_credentials.rs
@@ -52,17 +52,17 @@ pub const CLIENT_SUPPORTED_AUTH_METHODS: &[OAuthClientAuthenticationMethod] = &[
 
 /// A function that takes a map of claims and a signing algorithm and returns a
 /// signed JWT.
-pub type JwtSigningFn = fn(HashMap<String, Value>, JsonWebSignatureAlg) -> Result<String, BoxError>;
+pub type JwtSigningFn =
+    dyn Fn(HashMap<String, Value>, JsonWebSignatureAlg) -> Result<String, BoxError> + Send + Sync;
 
 /// The method used to sign JWTs with a private key.
-#[derive(Clone)]
 pub enum JwtSigningMethod {
     /// Sign the JWTs with this library, by providing the signing keys.
     #[cfg(feature = "keystore")]
     Keystore(Keystore),
 
     /// Sign the JWTs in a callback.
-    Custom(JwtSigningFn),
+    Custom(Box<JwtSigningFn>),
 }
 
 impl JwtSigningMethod {
@@ -75,8 +75,14 @@ impl JwtSigningMethod {
 
     /// Creates a new [`JwtSigningMethod`] from a [`JwtSigningFn`].
     #[must_use]
-    pub fn with_custom_signing_method(signing_fn: JwtSigningFn) -> Self {
-        Self::Custom(signing_fn)
+    pub fn with_custom_signing_method<F>(signing_fn: F) -> Self
+    where
+        F: Fn(HashMap<String, Value>, JsonWebSignatureAlg) -> Result<String, BoxError>
+            + Send
+            + Sync
+            + 'static,
+    {
+        Self::Custom(Box::new(signing_fn))
     }
 
     /// Get the [`Keystore`] from this [`JwtSigningMethod`].
@@ -101,7 +107,6 @@ impl JwtSigningMethod {
 
 /// The credentials obtained during registration, to authenticate a client on
 /// endpoints that require it.
-#[derive(Clone)]
 pub enum ClientCredentials {
     /// No client authentication is used.
     ///
diff --git a/crates/oidc-client/tests/it/main.rs b/crates/oidc-client/tests/it/main.rs
index e5da17ec..7b27876b 100644
--- a/crates/oidc-client/tests/it/main.rs
+++ b/crates/oidc-client/tests/it/main.rs
@@ -130,7 +130,7 @@ fn id_token(issuer: &str) -> (IdToken, PublicJsonWebKeySet) {
 fn client_credentials(
     auth_method: OAuthClientAuthenticationMethod,
     issuer: &Url,
-    custom_signing: Option<JwtSigningFn>,
+    custom_signing: Option<Box<JwtSigningFn>>,
 ) -> ClientCredentials {
     match auth_method {
         OAuthClientAuthenticationMethod::None => ClientCredentials::None {
diff --git a/crates/oidc-client/tests/it/types/client_credentials.rs b/crates/oidc-client/tests/it/types/client_credentials.rs
index f4b78caf..55c2cc5c 100644
--- a/crates/oidc-client/tests/it/types/client_credentials.rs
+++ b/crates/oidc-client/tests/it/types/client_credentials.rs
@@ -372,7 +372,7 @@ async fn pass_private_key_jwt_with_custom_signing() {
     let client_credentials = client_credentials(
         OAuthClientAuthenticationMethod::PrivateKeyJwt,
         &issuer,
-        Some(|_claims, _alg| Ok("fake.signed.jwt".to_owned())),
+        Some(Box::new(|_claims, _alg| Ok("fake.signed.jwt".to_owned()))),
     );
     let token_endpoint = issuer.join("token").unwrap();
     let mut rng = rand_chacha::ChaCha8Rng::seed_from_u64(42);
@@ -441,7 +441,7 @@ async fn fail_private_key_jwt_with_custom_signing() {
     let client_credentials = client_credentials(
         OAuthClientAuthenticationMethod::PrivateKeyJwt,
         &issuer,
-        Some(|_claims, _alg| Err("Something went wrong".into())),
+        Some(Box::new(|_claims, _alg| Err("Something went wrong".into()))),
     );
     let token_endpoint = issuer.join("token").unwrap();
     let mut rng = rand_chacha::ChaCha8Rng::seed_from_u64(42);