factor variables may not contain noninteger values margin stata

Question

I am trying to estimate the marginal effects of my xtlogit model in stata, which looks like this:

xtlogit onset c.l.log_welfarespending##c.l.ethnic_groups l.gdplog, re vce(robust)

The model itself runs smoothly and there are no complications at all. When I try to estimate the marginal effects:

margins log_welfarespending, at(l.ethnic_groups(= (0, 6))

I receive log_welfarespending: factor variables may not contain noninteger values. From what I have read so far sometimes Stata needs a specification on whether the variables are categorical or continuous. When I change the code to:

c.margins log_welfarespending, at(l.ethnic_groups(= (0, 6))

The error changes to only factor variables and their interactions are allowed. Any suggestions how to solve the problem?