[firebase-br] Comparar campo Blob numa trigger !

[Gugui]

ola pessoal, qdo comparo os campos Blob recebo este erro !

if( old.Obs != new.Obs ) then...


SQL Message : -901
Unsuccessful execution caused by system error that does not preclude 
successful execution of subsequent statements

Engine Code    : 335544382
Engine Message :
BLOB not found


Uso Firebird 2.1

Qual a melhor maneira de se fazer isso ?
grato qq ajuda