Sorry for my english :)

When call method $("#wizard").jWizard("next"); on last page, then blank page is opening, and any buttons are disabled. I use that check, to prevent this situation.

step : function($step) {
  var wizard = this, dfd = $.Deferred();
  if (typeof $step === "number") {
    $step = this.$steps.eq($step)
  }

  >>if(this.$steps.index($step)==-1)
    >>return;

May be it may be useful to anybody.