Not getting tha sum of columns of currency fields in output

Hi experts,

When I am running my program, for some columns like quantity i am getting the sum of that column when i choose the column and choose the sum button.

But when I am choosing any column like currency,and then choosing the sum button,It is not giving me the sum of the values.

It is giving an error .i.e. "DESIRED OPERATIONCANNOT BE PERFORMEDFOR COLUMN 'UNIT VOLUME IN GBP' "

UNIT VOLUME IN GBP is the column name.

Can u please suggest me what to do

Thanks