i have the following code which works sometimes and does not work at other times without giving any error.

set excl on
use tbsale2 in 0 excl
sele tbsale2
if type('tbsale2.ngroupname')='U'
alter table tbsale2 add ngroupname N(6)
endif

since it works sometimes i assume the code is correct, a read only on the table should give me an error, however i get no error and the field does not get added, i am a bit lost here, any suggestions?