diff --git a/Cargo.lock b/Cargo.lock
index 0556d063..34b1afb2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1215,6 +1215,7 @@ dependencies = [
  "parking_lot",
  "serde",
  "serde_yaml",
+ "thiserror",
  "tokio",
  "tokio-util",
  "url",
diff --git a/opcua-core/Cargo.toml b/opcua-core/Cargo.toml
index 34bfc79b..cdb50b30 100644
--- a/opcua-core/Cargo.toml
+++ b/opcua-core/Cargo.toml
@@ -28,6 +28,7 @@ url = { workspace = true }
 
 opcua-types = { path = "../opcua-types" }
 opcua-crypto = { path = "../opcua-crypto" }
+thiserror = { workspace = true }
 
 
 [lints.rust]
diff --git a/opcua-core/src/errors.rs b/opcua-core/src/errors.rs
new file mode 100644
index 00000000..6c9a854c
--- /dev/null
+++ b/opcua-core/src/errors.rs
@@ -0,0 +1,17 @@
+// OPCUA for Rust
+// SPDX-License-Identifier: MPL-2.0
+
+//!  Rust OpcUa specific errors
+
+use opcua_types::VariantScalarTypeId;
+use thiserror::Error;
+
+/// Rust OpcUa specific errors
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub enum OpcUAError {
+    #[error("Received an unexpected variant type")]
+    UnexpectedVariantType(Option<VariantScalarTypeId>),
+    #[error("The requested namespace does not exists")]
+    NamespaceDoesNotExist(String),
+}
diff --git a/opcua-core/src/lib.rs b/opcua-core/src/lib.rs
index 14f8ad6e..a4b28a32 100644
--- a/opcua-core/src/lib.rs
+++ b/opcua-core/src/lib.rs
@@ -64,6 +64,7 @@ pub mod constants {
 
 pub mod comms;
 pub mod config;
+pub mod errors;
 pub mod handle;
 
 pub mod messages;
diff --git a/samples/client.conf b/samples/client.conf
index 4f87d09a..ea41b25c 100644
--- a/samples/client.conf
+++ b/samples/client.conf
@@ -9,13 +9,6 @@ verify_server_certs: true
 pki_dir: ./pki
 preferred_locales: []
 default_endpoint: sample_none
-user_tokens:
-  sample_user:
-    user: sample1
-    password: sample1pwd
-  sample_user2:
-    user: sample2
-    password: sample2pwd
 endpoints:
   sample_basic128rsa15:
     url: opc.tcp://127.0.0.1:4855/
@@ -37,6 +30,14 @@ endpoints:
     security_policy: None
     security_mode: None
     user_token_id: ANONYMOUS
+user_tokens:
+  sample_user:
+    user: sample1
+    password: sample1pwd
+  sample_user2:
+    user: sample2
+    password: sample2pwd
+channel_lifetime: 60000
 decoding_options:
   max_message_size: 327675
   max_chunk_count: 5
@@ -65,10 +66,9 @@ publish_timeout:
 min_publish_interval:
   secs: 0
   nanos: 100000000
-session_timeout: 60000
 performance:
   ignore_clock_skew: false
   recreate_monitored_items_chunk: 1000
-session_name: Rust OPC UA Client
-channel_lifetime: 60000
 recreate_subscriptions: true
+session_name: Rust OPC UA Client
+session_timeout: 60000