I am using an unbalanced firm-level panel data. I am regressing the market share of the firms on a set of independent variables that account for the firm characteristics.
I am running a fixed-effect model. My panel variable is the firm id.
I would like to cluster my error by sector to get rid of any serial correlation between firms in the same sector.

I am using this command
xtreg lshare ltariff ltfp lwages lage trade, fe vce (cluster sector)

However, I got this error "panels are not nested within clusters".

What should I do to correct for this?

Thanks in advance.