Lost forecast ensemble keys when applying cdo function
Good morning,
I m working with ECMWF ensemble files S7E . I m trying to apply a simple landsea mask to reduce the amount of data to load in our production system.
This the function to apply my grib mask :
/opt/cdo-1.7.0/bin/cdo mul S7E_LSM_100km.grib S7E04110000041103001_100v.grb S7E04110000041103001_100v_masked.grb.tmp
This step is fine, my result field is correctly masked, but we are losing forecast ensemble keys :-( which is mandatory is our system.
Here is an example: left panel is my masked field and right one is my original field: 
Do you know if I m missing something here to keep grib keys (ie pertubation number) in my result field ?
Many thanks for your help !
Guillaume