CDO

Hello :)
I need to merge a variable to an existent dataset, which I use to run some scripts after.
The problem I have is that the rest of the variables are written in I16 format, and the new one that I need to include is F32.
But when I try 'cdo -b I16 copy file ofile' I get the following:
cdo copy: 0%cdf_put_att_double: ncid = 131072 varid = 3 att = _FillValue val = 1e+20

Error (cdf_put_att_double): NetCDF: Numeric conversion not representable

When I remove the "I" from that formula it works, like: cdo -b 16 copy file ofile
But when I do sinfon ofile I still get Dtype as F32.
I have been checking my scripts until losing my eyes, and I don't know whats' happening, this looks like the only solution left, to change that F32 to I16. Could you help me?
Thank you so so much