How to make radio inputs required?

I have radio inputs in a form for the user to select their gender. How do I make gender a required input (user must select either female or male or they get a prompt as per the usual required).

I tried adding required as shown below but it doesn't work (user can continue without selecting either female or male).

              <div class="form-group btn-group" data-toggle="buttons">
                <label class="btn btn-default btn-lg">
                  <input type="radio" name="gender" value="Female" required><i class="fa fa-female"></i> Female
                </label>
                <label class="btn btn-default btn-lg">
                  <input type="radio" name="gender" value="Male" required><i class="fa fa-male"></i> Male
                </label>
              </div>