st: could not calculate numerical derivatives -- flat or discontinuous region encountered

[Usman Gilani <ujgilani@gmail.com>]

Dear members,
I'm trying to run the following non-linear regression using gmm in state 12. but getting the error message 
and secondly do i have to tell stata that i'm doing the estimation using panel data however before running this equation i'm using this command

xtset panelvar timevar

and after that i'm running this equation.

gmm (({a0}*{a1}*w1*DP+(1-{a0}*DP)*lr1+{a0}*{a2}*DP*w2+{a0}*{a3}*DP*w3+{a0}*{a4}*DP*y1+{a0}*{a
> 5}*DP*y2+{a0}*{a6}*DP*y3)), xtinstruments(w1 w2 w3 y1 y2 y3 DP lr1, lags(1/2)) winitial(xt L)
warning: 472 missing values returned for equation 1 at initial values

ERROR message:

"Step 1
could not calculate numerical derivatives -- flat or discontinuous region encountered
could not calculate numerical derivatives -- flat or discontinuous region encountered
r(430);"

any suggestions how to avoid it or what changes should i make to my data. I have a panel data for 80 firms over the period of 20 years. 

thanks in advance.

regards
Gilani
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