assign ..... <fs>statement is failing sometime

assign (t_kbert) to <fs>

<fs> = outtab-kbert.

above is the code, in which assign statement is failing sometime and sometimes succesful.

times when it is failing it goes to short dump since <fs> is unassinged.

at any time t_kbert is having the value.

Pls, help me in solving this.

Thanks