diff --git a/stun/src/agent.rs b/stun/src/agent.rs
index 5938c7917..1ddad2b7b 100644
--- a/stun/src/agent.rs
+++ b/stun/src/agent.rs
@@ -2,7 +2,7 @@
 mod agent_test;
 
 use std::collections::HashMap;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 
 use rand::Rng;
 use tokio::sync::mpsc;
@@ -34,7 +34,7 @@ pub struct Agent {
     /// data races via unexpected concurrent access.
     transactions: HashMap<TransactionId, AgentTransaction>,
     /// all calls are invalid if true
-    closed: bool,
+    closed: Mutex<bool>,
     /// handles transactions
     handler: Handler,
 }
@@ -118,7 +118,7 @@ impl Agent {
     pub fn new(handler: Handler) -> Self {
         Agent {
             transactions: HashMap::new(),
-            closed: false,
+            closed: Mutex::new(false),
             handler,
         }
     }
@@ -126,7 +126,8 @@ impl Agent {
     /// stop_with_error removes transaction from list and calls handler with
     /// provided error. Can return ErrTransactionNotExists and ErrAgentClosed.
     pub fn stop_with_error(&mut self, id: TransactionId, error: Error) -> Result<()> {
-        if self.closed {
+        let closed = self.closed.lock().unwrap();
+        if *closed {
             return Err(Error::ErrAgentClosed);
         }
 
@@ -135,6 +136,8 @@ impl Agent {
             .remove(&id)
             .ok_or(Error::ErrTransactionNotExists)?;
 
+        drop(closed);
+
         if let Some(handler) = &self.handler {
             handler.send(Event {
                 event_type: EventType::Callback(t.id),
@@ -146,12 +149,15 @@ impl Agent {
 
     /// process incoming message, synchronously passing it to handler.
     pub fn process(&mut self, message: Message) -> Result<()> {
-        if self.closed {
+        let closed = self.closed.lock().unwrap();
+        if *closed {
             return Err(Error::ErrAgentClosed);
         }
 
         self.transactions.remove(&message.transaction_id);
 
+        drop(closed);
+
         let e = Event {
             event_type: EventType::Callback(message.transaction_id),
             event_body: Ok(message),
@@ -167,7 +173,8 @@ impl Agent {
     /// close terminates all transactions with ErrAgentClosed and renders Agent to
     /// closed state.
     pub fn close(&mut self) -> Result<()> {
-        if self.closed {
+        let mut closed = self.closed.lock().unwrap();
+        if *closed {
             return Err(Error::ErrAgentClosed);
         }
 
@@ -181,7 +188,7 @@ impl Agent {
             }
         }
         self.transactions = HashMap::new();
-        self.closed = true;
+        *closed = true;
         self.handler = noop_handler();
 
         Ok(())
@@ -192,7 +199,8 @@ impl Agent {
     ///
     /// Agent handler is guaranteed to be eventually called.
     pub fn start(&mut self, id: TransactionId, deadline: Instant) -> Result<()> {
-        if self.closed {
+        let closed = self.closed.lock().unwrap();
+        if *closed {
             return Err(Error::ErrAgentClosed);
         }
         if self.transactions.contains_key(&id) {
@@ -217,7 +225,8 @@ impl Agent {
     ///
     /// It is safe to call Collect concurrently but makes no sense.
     pub fn collect(&mut self, deadline: Instant) -> Result<()> {
-        if self.closed {
+        let closed = self.closed.lock().unwrap();
+        if *closed {
             // Doing nothing if agent is closed.
             // All transactions should be already closed
             // during Close() call.
@@ -240,6 +249,8 @@ impl Agent {
             self.transactions.remove(id);
         }
 
+        drop(closed);
+
         for id in to_remove {
             let event = Event {
                 event_type: EventType::Callback(id),
@@ -255,7 +266,8 @@ impl Agent {
 
     /// set_handler sets agent handler to h.
     pub fn set_handler(&mut self, h: Handler) -> Result<()> {
-        if self.closed {
+        let closed = self.closed.lock().unwrap();
+        if *closed {
             return Err(Error::ErrAgentClosed);
         }
         self.handler = h;