Can't total numc in ALV tree

Hi

I am having trouble calculating a column in my ALV tree.

I have a z table that I am getting my data from. The table has a column of type numc. I put the data into an internal table with a few calculations done to it. I use the internal table for my ALV tree. The problem is that i can not calculate totals for this column. I have another column that is type DEC but it calculates ok.