Unable to download depth-continuous data with llcreader

[kaka22acmilan]

Hello,
I'm a beginner of Python and xmitgcm,and I want to download deep-continuous data by llcreader,but if only set k_levels in model.get_dataset just like
model = llcreader.ECCOPortalLLC4320Model() ds = model.get_dataset(varnames=['U','V','W','Eta','Theta','Salt'], k_levels=[0, 1, 2, 3],iter_start=10368, iter_stop=10513, iter_step=144)# k is vertical level filePath = dirPath + '/klevel_3.nc' region_slice = {'i_g':slice(900,2250),'j':slice(3600,4320),\ 'i':slice(900,2250),'j_g':slice(3600,4320),\ 'face':slice(4,5)} region = ds.isel(**region_slice) region.to_netcdf(filePath, mode='w')
it will raise ValueError: Seek before start of file.
sometimes I changed my code as
region_slice = {'i_g':slice(900,2250),'j':slice(3600,4320),\ 'i':slice(900,2250),'j_g':slice(3600,4320),\ 'face':slice(4,5),'k':slice(0,4)}
another error is displayed.However, when I set discontinuous k_levels such as ds = model.get_dataset(varnames=['U','V','W','Eta','Theta','Salt'], k_levels=[1, 10, 40],iter_start=10368, iter_stop=10513, iter_step=144),it is no problem.I can't figure out the relationship between z and z_u or z_p1.
Thanks for your help.