[amibroker] not sure why i get this subscript out of range error

[murthysuresh]

My logic finds the barcount and then tries to access the bar which is 
barcount -10. Not sure why i get this error. i have enough history info 
to be able to see (barcount-10)th  value.

ThisIsLastBar = LastValue( BarIndex() ); 
y0=LastValue(ADX());

y1=Ref(ADX(),3);

WriteVal(y0) + "\n";
WriteVal(y1[ThisIsLastBar-10])   + "\n";

WriteVal(y1[ThisIsLastBar-1
--------------------------^

Error 10.
Subscript out of range.
You must not access array elements outside 0..(BarCount-1) range.