一个关于IsEmpty的问题

'当allAddress为空的时候,提示"for i=1 to length"这一行出错,无效使用null.
'当allAddress不为空的时候就没有问题了.



if not IsEmpty(allAddress) then
response.write "<td width='110'>"
length=len(allAddress)
address=""
for i=1 to length
temp=mid(allAddress,i,1)
if temp<>","then
address=address+temp
else
response.write "<a href='"&address&"' target='_blank'>"
response.write "<img border='0' src='"
response.write address&"' width='109'><br><br></a>"
address=""
end if
next
response.write "<a href='"&address&"' target='_blank'>"
response.write "<img border='0' src='"
response.write address&"' width='109'></a>"
response.write "</td>"
end if