Assigning '(SAPMM07M)MSEG-PRCTR'

Hi Experts.

I am working in 'EXIT_SAPMM07M_001'.

I want to assign a value into '(SAPMM07M)MSEG-PRCTR'.

My code:

Header 1 DATA: BP TYPE I. "Break Point
FIELD-SYMBOLS: <PRCTR> TYPE MSEG-PRCTR.

BP = 0.

ASSIGN '(SAPMM07M)MSEG-PRCTR' TO <PRCTR>.
<PRCTR> = 'TEST_DUMMY'.

BP = 1.

Error returns:

"'(SAPMM07M)MSEG-PRCTR'" and "<PRCTR>" are type-incompatible

If I change the TYPE of <PRCTR> to ANY, error returns:

"Error at assignment : Overwritten protected field."

May I ask for some hints?

BR,

King