Unable to plot phase noise

Hi,

After running pnoise: jitter analysis, I wanted to plot the phase noise by choosing:

Direct Plot Form => pnoise jitter => Phase Noise => dBc => Plot

I got some error messages shown as follow:

*Error* flip: can't handle flip(nil)
*Error* flip: can't handle flip(nil)
*Error* flip: can't handle flip(nil)
*Error*   Evaluating expression (_drplPmNoise(?modifier "dBc" ?dsb
          t)).
*Error* ("abs" 0 t nil ("*Error* abs: can't handle abs(nil)"))
*Error*   Trying to plot expression <_drplPmNoise(?modifier "dBc"
          ?dsb t)>, which does not evaluate to an object that can be
          plotted, like waveform or parametric wave. Please refer to
          the Wavescan User Guide to see what all types of objects
          can be plotted in Wavescan. Expression which evaluates to
          only those type(s) of objects can be plotted.

I could get the plot when I choose:

 Direct Plot Form => pnoise => Phase Noise => Plot

May I know what is the problem?

 Thanks.