GFS winds

[jgcmeteo]

ECMWF grib files are recommended, but they are extremely expensive for real time data or forecasts.
I have tried with GFS and windfield = otop.wind.read_grib(fgrib) seems to work fine and creates a pandas dataframe with realistic values. However flight = optimizer.trajectory(objective="fuel") fails with the following error:
File "/.../wind.py", line 57, in
eval(f, {}, {"x0": x, "x1": y, "x2": h}) * c
File "", line 1, in
NameError: name 'x' is not defined

Any idea about what may be wrong?