[error display in word]

hi,
when i mail merge using doc as my external source i made a field constant
using " if then else" from the 'insert word field ' tool.My result was
working fine.

requirement:
when i enter a replacement text other than what i had mentioned in the "if
then else " case i should get an error as we get in excel("data.. validation
.. show errors ).But its not prompting me for errors ...please help me on this
..

regards,
leo