How to add validation rule dynamically to checkbox array?

I am dynamically appending input fields to div. I wish to check if at least one checkbox is checked, for that I am adding JQuery validation rule dynamically to the array of input checkbox by using class. HTML:

<input type="checkbox" name="ans[]" value="opt_' + x + '" class="answer_check">

JavaScript:

 $(".answer_check").rules("add", {required: true, messages: {required: "Please select correct answer/s."}});

I am getting following error in console.

Syntax error, unrecognized expression: label[for='ans[]'], label[for='ans[]'] *, #ans[]-error