Calculation error in postion function

hi all
I have this error while iam using a position(..) function with repeated group. the problem appear when i deselect choices from multiple choice question.

Calculation Error: Error in calculation for /data/diagnosis1/HD/position_num1 Unable to evaluate position on /data/diagnosis1[1]/HD[1], which is empty.

the scenario as follow:

  • Multiple choice question
    -repeated group depended on the choices
    -every group has a calculation for position
    -if i select normal choices its ok, but if i select then deselect the problem appear.