Problem when using more than one DatePicker

Problem when using more than one DatePicker

Hello, I'm having trouble with ui.datepicker.
I have the following code:
  1. <input type="text" name="dtFrom" class="dpk">
  2. <input type="text" name="dtUntil" class="dpk">
  3. <script type="text/javascript">
  4.   $(":text.dpk").datepicker();
  5. </script>
When I click [dtFrom] its respective datepicker opens and if I don't select any date, clicking straight on [dtUntil], [dtUntil] opens with no animation, closes with animation, and opens with animation.
That's wrong, [dtFrom] should be closed and [dtUntil] should be opened.

Am I doing something wrong?

Thanks in advance.