SQP Query - calculate Total on Extra field

Hi SAP Experts,

Greetings to all...

I wish to ask following question:

I have created a SAP Query and I have added Extra fields in it. I am performing some calculation in it (using code). When I try to click "TOTAL" after selecting the extra column I see the following message:

Desired operation cannot be performed for column

Please let me know if i have missed any setting.

Thanking you in advance.

Regards,

UK.