Fast Formula retrieving Date format

Summary:

Hello experts,

we built the following formula to retrieve a date to be used in compensation plan in date custom field.

However, when testing it the attached error appears.

L_ASG_ID = get_context(HR_ASSIGNMENT_ID, -1)
RETURN_VALUE=' '
L_ASG_ID_char=TO_CHAR(L_ASG_ID)
RETURN_VALUE = GET_VALUE_SET('ST_JOB_GRADE_DATE','|=ASSIGNMENT_ID='''||l_asg_id_char||'''')
l_comp_date = to_date(RETURN_VALUE,'dd/mm/yyyy')
RETURN l_comp_date

The value set returns the date in text format but in FF I need to convert it into Date format.