diff --git a/api/client.go b/api/client.go
index de21dc5..023499f 100644
--- a/api/client.go
+++ b/api/client.go
@@ -1,6 +1,7 @@
 package api
 
 import (
+	"fmt"
 	"sync"
 
 	"github.com/eosswedenorg/thalos/api/message"
@@ -63,13 +64,16 @@ func (c *Client) hbHandler(payload []byte) {
 	c.OnHeartbeat(hb)
 }
 
-func (c *Client) Subscribe(channel Channel) {
+func (c *Client) Subscribe(channel Channel) error {
 	var handler handler
 
-	if HeartbeatChannel.Is(channel) {
+	switch t := channel.Type(); t {
+	case HeartbeatChannel.Type():
 		handler = c.hbHandler
-	} else {
+	case ActionChannel{}.Channel().Type():
 		handler = c.actHandler
+	default:
+		return fmt.Errorf("invalid channel type. %s", t)
 	}
 
 	// Start a worker for this channel.
@@ -78,6 +82,8 @@ func (c *Client) Subscribe(channel Channel) {
 		defer c.wg.Done()
 		c.worker(channel, handler)
 	}()
+
+	return nil
 }
 
 func (c *Client) Run() {
diff --git a/api/client_test.go b/api/client_test.go
new file mode 100644
index 0000000..2a1061b
--- /dev/null
+++ b/api/client_test.go
@@ -0,0 +1,26 @@
+package api
+
+import (
+	"testing"
+)
+
+func TestClient_Subscribe(t *testing.T) {
+	tests := []struct {
+		name    string
+		channel Channel
+		wantErr bool
+	}{
+		{"Channel", Channel{}, true},
+		{"ActionChannel", ActionChannel{}.Channel(), false},
+		{"HeartbeatChannel", HeartbeatChannel, false},
+		{"TransactionChannel", TransactionChannel, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := Client{}
+			if err := c.Subscribe(tt.channel); (err != nil) != tt.wantErr {
+				t.Errorf("Client.Subscribe() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}