Interactive chart throws error message when I reduce data sources, how to avoid?

[Butcher1]

On the Summary sheet in A19:A20 I have drop down menus that select a row label and the associated data is displayed in the chart. I am using this formula,
=OFFSET(INDEX(Chart_Options,MATCH(Summary!$A$20,Chart_Options,0)),0,1,1,(15-COUNTBLANK(Summary!$B$5:$P$5)))
as a defined name and is being entered in as the series values.

The problem is if I decide I want to only see one set of data in the chart and erase the text in A20 I get an error message. Everything is still functional but I want it so the error message doesn't get displayed. How do I work around this?