syntax for using a variable in an array

[tob at b-e-s-t.com]

I want to loop through an invoice file and find invoice numbers that have been used and thus invoice numbers that have not been used.
The prc below will do it but i do not have the syntax for using a variable in an array correct.
Thanks

::dim ni(100000)(8,.0):
::rn(8,.0)="1":
::iv(5,.0)=""
nxinv:::
::lookup inv = muinv  r=rn   -n:
:not inv:goto fini:
::iv=inv(2):
::ni(iv)="1":  LINE THAT ERRORS
::rn=rn+"1":
::getnext inv:
::goto nxtinv:
fini:::
:' output total left::