Error In Select Query

i have select query:

select distinct s.stype, s.snumber, r.custnumber, r.custname, r.creditdate,

case when (r.creditdate >= dateAdd(m, -3, convert(datetime,convert(varchar(10), getdate(), 112), 112))
AND r.creditdate < dateAdd(d, 1, convert(datetime,convert(varchar(10), getdate(), 112), 112)))
then r.creditnumber
else replace(r.creditnumber,substring(r.creditnumber,1,len(r.creditnumber)-4),'xxxx xxxx xxxx
')
end as creditcard from creditcardtable r

join salestable s

on s.salesdate = r.creditdate

where (r.creditnumber <> '') AND (NOT (ISNULL(r.creditnumber, '999') = '999'))

order by r.creditdate desc

first time i execute this query i got an error like:
An error occurred while executing batch. Error message is: Internal connection fatal error.

second time i execute this query i got an error like:
An error occurred while executing batch. Error message is: Non-negative number required.
Parameter name: count

can anyone tell me what's the reason? and any solution for it?

thanks.