Pertmaster Schedule Warning - Undeflow

Long time since I posted anything on here, but stuck at the moment

Have been carrying out schedule risk analysis on a schedule with multilpe calendars, 5 day, 6 day and weekends only.  Pertmaster runs and shows up an error message stating check Schedule Warnings against activities. 

The error message states

"Warning-some activities have tired to be schedule before or after the calendar range.  These tasks have been forced within the calendar range. To see which tasks have caused this, check Schedule Warnings column in the Gantt Chart"

 

Have looked at the column information and the Schedule Warning info shows the term "Underflow" against the activities with weekend only calenders

The risk model does perform as expected, having stpped through the analysis, owever curous as to the error message, no mention of it in the help files!