Assigning values by "margin at" with covariates

Hello All,

Is there any way that I can assign a set of values to a covariate with margins command by using a variable already defined in the data?
More specifically I am running the following command:

'margin if S13==1&index<meanindex, at (index = meanindex) post'

and getting this:

"'meanindex' found where number expected"

There are 60 categories for S13 (from 1 to 60) and "meanindex" is the mean value of "index" for each S13 calculated by egen.

Any help much appreciated!

Thanks a lot
--Yigit