xtivreg , first failes with "conformability error" r(503)

Dear Readers

xtivreg without "first" runs fine, but fails when I add the "first" option with "conformability error" r(503)

Using "set trace on" the error appears at:

Code:

est repost b=`bw', rename findomitted buildfvinfo
= est repost b=__00008Q, rename findomitted buildfvinfo
conformability error
di
di as text "First-stage within regression"
`vv' xtreg , level(`level') `diopts'
}
}
    end    xtivreg.Estimate    ---
        end xtivreg    ---
r(503);

Any ideas?

Windows 10 pro up to date and Stata 16.1 29/9/2020

All help most appreciated.

Best wishes

Richard