Aleatory exception

Hi there,

I have a custom function where I execute a dynamic statement. I trap the 
exceptions, and sometimes I got the following error data:           SQLSTATE = 22P02, SQLERRM = invalid input syntax
forinteger: 
 
"NULL::int4"

For instance I got this for the following statement string:

Please explain what could be the reason ?

TIA,
Sabin