Vaadin 8 Checkbox Group select() doesn't work for array

Question

I have the code that creates a CheckBoxGroup (based on Vaadin 8 Sampler):

public void setPossibleAnswers(List <String> answerOptions) {       
   answerCheckBoxes = new CheckBoxGroup<>("", answersOptions);      
}

I'm using this method to select given items in a group:

public void setAnswer(String answer) {
    String[] answers = answer.split(",");
    answerCheckBoxes.select(answers);
}

But this code selects only the first element of the array. What I need is to select all elements. Does any body know where is my mistake?