Error : Enhancing an infotype

Hi,

Actually i want to add one field in infotype 9008 .

I followed below step for addition of field:

PM01 > enter infotype > Edit button > enter field in CI_9008 -> Activate it . After this when iam clicking on single screen-generate object for the the respective infotype , It throwing error saying ' infotype 9008 cant be enhanced '.

Kindly help me in this regard.

Thanks in Advance,

Shweta