Message pops up say can't divide by zero

[Robert W Cummings <robertwc@xxxxxxxxxxxxxxxxxx>]

Input: Price(C),Length(18),SDev(2),upper(100),Lower(-100);
Plot1(((100*(Price-$Lowerband(Price,Length,SDev))/($Upperband(Price,Length,S
Dev)-$Lowerband(Price,Length,SDev)))-50)*2,"Place");

Plot2(upper,"Upper");
Plot3(lower,"Lower");
IF CheckAlert Then Begin
  IF Plot1 Crosses Above Plot2 or Plot1 Crosses Below Plot2
  or Plot1 Crosses Above Plot3 or Plot1 Crosses Below Plot3
  or Plot2 Crosses Above Plot3 or Plot2 Crosses Below Plot3
  Then Alert = TRUE;
End;

Can't figure what its needs without messing up what the original author was
attempting to do here.
Anybody see the problem it will verify.

Thanks in Advance

Robert