diff --git a/server.go b/server.go
index 3a44e5c..2d6d181 100644
--- a/server.go
+++ b/server.go
@@ -256,14 +256,15 @@ func (srv *Server) Run(handler Handler) error {
 // Start returns any error encountered during server boot time.
 // If the server has already been stopped, ErrServerStopped is returned.
 func (srv *Server) Start(handler Handler) error {
-	// TODO: Retrun error if handler is nil
+	if handler == nil {
+		return fmt.Errorf("asynq: server cannot run with nil handler")
+	}
 	switch srv.ss.Status() {
 	case base.StatusRunning:
 		return fmt.Errorf("asynq: the server is already running")
 	case base.StatusStopped:
 		return ErrServerStopped
 	}
-	// TODO: Return error if cannot connect to Redis
 	srv.ss.SetStatus(base.StatusRunning)
 	srv.processor.handler = handler
 
diff --git a/server_test.go b/server_test.go
index 9c7188e..cb56ad4 100644
--- a/server_test.go
+++ b/server_test.go
@@ -50,6 +50,41 @@ func TestServer(t *testing.T) {
 	srv.Stop()
 }
 
+func TestServerErrServerStopped(t *testing.T) {
+	srv := NewServer(RedisClientOpt{Addr: ":6379"}, Config{})
+	handler := NewServeMux()
+	if err := srv.Start(handler); err != nil {
+		t.Fatal(err)
+	}
+	srv.Stop()
+	err := srv.Start(handler)
+	if err != ErrServerStopped {
+		t.Errorf("Restarting server: (*Server).Start(handler) = %v, want ErrServerStopped error", err)
+	}
+}
+
+func TestServerErrNilHandler(t *testing.T) {
+	srv := NewServer(RedisClientOpt{Addr: ":6379"}, Config{})
+	err := srv.Start(nil)
+	if err == nil {
+		t.Error("Starting server with nil handler: (*Server).Start(nil) did not return error")
+		srv.Stop()
+	}
+}
+
+func TestServerErrServerRunning(t *testing.T) {
+	srv := NewServer(RedisClientOpt{Addr: ":6379"}, Config{})
+	handler := NewServeMux()
+	if err := srv.Start(handler); err != nil {
+		t.Fatal(err)
+	}
+	err := srv.Start(handler)
+	if err == nil {
+		t.Error("Calling (*Server).Start(handler) on already running server did not return error")
+	}
+	srv.Stop()
+}
+
 func TestGCD(t *testing.T) {
 	tests := []struct {
 		input []int