substr or char_length problem

Dear ALL

I have a problem with function substr or char_length or both. I guery A2 and 
it works fine. But sometimes gives 'ERROR:  negative substring length not 
allowed'. When I test many many times with diffrent values, never gives 
error. Sample table and query below.



A1              A2
-------------------
1               1957
2               197
3               19
4
5               NULL
6               1
7               195


Select * from tbl_xxx where 
tbl_xxx.A2=substr('196895588454554545454',0,char_length(tbl_xxx.A2)+1);


Thanks
Fatih Cerit