Microsoft VBScript runtime error '800a005e'

[princesuhaib]

I am getting this error :

Microsoft VBScript runtime error '800a005e'

Invalid use of Null

/Admin/function.inc, line 12


this is my lines from 6-17

for i = 1 to 10000
begstr = "$r$"
enstr= "$r$"
beg = Instr(1,qstr,begstr,1) + len(begstr)

if beg = len(begstr) then exit for
en = Instr(beg + 1,qstr,enstr,1)
if en = 0 then en = len(qstr) + 1

rstr = mid(qstr,beg,en - beg)
qstr=replace(qstr,"$r$" & rstr,"")
resp(i)=right(rstr,len(rstr)-instr(1,rstr,"|",1))

the red is line 12 please help me correct this thank you very much!
Respectfully
Suhaib