diff --git a/builtin.go b/builtin.go
index e7ccc19..12257ca 100644
--- a/builtin.go
+++ b/builtin.go
@@ -113,7 +113,9 @@ func handlePING(c *Client, e Event) {
 }
 
 func handlePONG(c *Client, e Event) {
+	c.conn.mu.Lock()
 	c.conn.lastPong = time.Now()
+	c.conn.mu.Unlock()
 }
 
 // handleJOIN ensures that the state has updated users and channels.