How do use 'do_sum' in fieldcat?

Hello,

I am trying to use 'do_sum' in a fieldcat so I can sum up the column values of my internal table.
Here's what I have:

      i_fieldcat-fieldname = 'VALUE'.
     i_fieldcat-coltext = 'Value'.
     i_fieldcat-ref_table = 'i_tab'.
     i_fieldcat-ref_field = 'i_tab-value'.
     i_fieldcat-do_sum = 'X'.
     APPEND i_fieldcat TO t_fieldcat.

The problem is that the 'Sum' icon on the toolbar is disabled, so I can't do anything.

Thanks in advance.