When reading the value of the evaluation Class, the AKLAS is empty, what does that mean ?

I am using the function 'HR_99S_GET_EVAL_CLASS_VALUE' to read the evaluation class ,where:

molga = UN

lagrt = /559

eval_class = 18

key_date = sy-datum

the result:

value is empty and return code = 0.

what does it mean when the value is empty ?