diff --git a/internal/client.go b/internal/client.go
index f414bfd..ea2b731 100644
--- a/internal/client.go
+++ b/internal/client.go
@@ -6,6 +6,12 @@ import (
 	"time"
 )
 
+// we need safety timeout, to prevent send more than
+// one message in http request.
+const (
+	safetyTimeout = 1
+)
+
 // client is our user application handler.
 type client struct {
 	// map of topics
@@ -79,7 +85,9 @@ func (c *client) listen() {
 
 // handle will execute the topic handler method.
 func (c *client) handle(m message) {
-	c.topics[m.Topic](m.Data)
+	if handler, ok := c.topics[m.Topic]; ok {
+		handler(m.Data)
+	}
 }
 
 // close will terminate everything.
@@ -94,27 +102,29 @@ func (c *client) Publish(topic string, data []byte) error {
 		return err
 	}
 
-	time.Sleep(1 * time.Millisecond)
+	time.Sleep(safetyTimeout * time.Millisecond)
 
 	return nil
 }
 
 // Subscribe subscribes over broker.
 func (c *client) Subscribe(topic string, handler MessageHandler) {
+	// set a handler for given topic
+	c.topics[topic] = handler
+
+	// send an http request to broker server
 	err := c.network.send(encodeMessage(newMessage(Subscribe, topic, nil)))
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	time.Sleep(1 * time.Millisecond)
-
-	// set a handler for given topic
-	c.topics[topic] = handler
+	time.Sleep(safetyTimeout * time.Millisecond)
 }
 
+// Unsubscribe removes client from subscribing over a topic.
 func (c *client) Unsubscribe(topic string) {
 	_ = c.network.send(encodeMessage(newMessage(Unsubscribe, topic, nil)))
-	time.Sleep(1 * time.Millisecond)
+	time.Sleep(safetyTimeout * time.Millisecond)
 
 	// remove topic and its handler
 	delete(c.topics, topic)