How to set array element to null value

I'm trying to reset array element to null. but 3rd line of below snippet is giving the compilation error.

FOR indx_1 IN array_lower(X, 1)..array_upper(X, 1) LOOP

IF X[indx_1].REFERENCE_VALUE = 'ABC' THEN

X[indx_1].REFERENCE_VALUE:='';

END IF;

END LOOP;