false_easting = None: loading OK, but saving gives an error

Here is a example using a CORDEX file from ESGF:
iris.__version__                                                                                                                                                                
Out[25]: '2.3.0'

In [26]: c =  iris.load_cube('/our/own/local/directory/hierarchy/snw_EUR-11_ECMWF-ERAINT_evaluation_r1i1p1_CNRM-ALADIN63_v1_day_19790101-19801231.nc')                                                                                                                                                         

In [27]: c.coord('projection_x_coordinate')                                                                                                                                              
Out[27]: 
DimCoord(array([   0. ,   12.5,   25. ,   37.5,   50. ,   62.5,   75. ,   87.5,
        100. ,  112.5,  125. ,  137.5,  150. ,  162.5,  175. ,  187.5,
##   8>< snip    8>< snip
       5500. , 5512.5, 5525. , 5537.5, 5550. , 5562.5, 5575. , 5587.5,
       5600. , 5612.5, 5625. , 5637.5, 5650. ]), standard_name='projection_x_coordinate', units=Unit('km'), long_name='x coordinate of projection', var_name='x', coord_system=LambertConformal(central_lat=49.5, central_lon=10.5, false_easting=None, false_northing=None, secant_latitudes=(49.5,), ellipsoid=None))

In [28]: iris.save(c,'c.nc')                                                                                                                                                             
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-28-4160b1a355b0> in <module>
----> 1 iris.save(c,'c.nc')

~/.conda/envs/climix-devel-3/lib/python3.8/site-packages/iris/io/__init__.py in save(source, target, saver, **kwargs)
    406     # Single cube?
    407     if isinstance(source, iris.cube.Cube):
--> 408         saver(source, target, **kwargs)
    409 
    410     # CubeList or sequence of cubes?

~/.conda/envs/climix-devel-3/lib/python3.8/site-packages/iris/fileformats/netcdf.py in save(cube, filename, netcdf_format, local_keys, unlimited_dimensions, zlib, complevel, shuffle, fletcher32, contiguous, chunksizes, endian, least_significant_digit, packing, fill_value)
   2386         # Iterate through the cubelist.
   2387         for cube, packspec, fill_value in zip(cubes, packspecs, fill_values):
-> 2388             sman.write(cube, local_keys, unlimited_dimensions, zlib, complevel,
   2389                        shuffle, fletcher32, contiguous, chunksizes, endian,
   2390                        least_significant_digit, packing=packspec,

~/.conda/envs/climix-devel-3/lib/python3.8/site-packages/iris/fileformats/netcdf.py in write(self, cube, local_keys, unlimited_dimensions, zlib, complevel, shuffle, fletcher32, contiguous, chunksizes, endian, least_significant_digit, packing, fill_value)
    981 
    982         # Create the associated cube CF-netCDF data variable.
--> 983         cf_var_cube = self._create_cf_data_variable(
    984             cube, dimension_names, local_keys, zlib=zlib, complevel=complevel,
    985             shuffle=shuffle, fletcher32=fletcher32, contiguous=contiguous,

~/.conda/envs/climix-devel-3/lib/python3.8/site-packages/iris/fileformats/netcdf.py in _create_cf_data_variable(self, cube, dimension_names, local_keys, packing, fill_value, **kwargs)
   2131 
   2132         # Create the CF-netCDF grid mapping.
-> 2133         self._create_cf_grid_mapping(cube, cf_var)
   2134 
   2135         return cf_var

~/.conda/envs/climix-devel-3/lib/python3.8/site-packages/iris/fileformats/netcdf.py in _create_cf_grid_mapping(self, cube, cf_var_cube)
   1848                     cf_var_grid.latitude_of_projection_origin = cs.central_lat
   1849                     cf_var_grid.longitude_of_central_meridian = cs.central_lon
-> 1850                     cf_var_grid.false_easting = cs.false_easting
   1851                     cf_var_grid.false_northing = cs.false_northing
   1852 

netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Variable.__setattr__()

netCDF4/_netCDF4.pyx in netCDF4._netCDF4.Variable.setncattr()

netCDF4/_netCDF4.pyx in netCDF4._netCDF4._set_att()

TypeError: illegal data type for attribute b'false_easting', must be one of dict_keys(['S1', 'i1', 'u1', 'i2', 'u2', 'i4', 'u4', 'i8', 'u8', 'f4', 'f8']), got O

In [29]: !ncdump -h /our/own/local/directory/hierarchy/snw_EUR-11_ECMWF-ERAINT_evaluation_r1i1p1_CNRM-ALADIN63_v1_day_19790101-19801231.nc                                                                                                                                                                     
netcdf snw_EUR-11_ECMWF-ERAINT_evaluation_r1i1p1_CNRM-ALADIN63_v1_day_19790101-19801231 {
dimensions:
    y = 453 ;
    x = 453 ;
    nvertex = 4 ;
    time = UNLIMITED ; // (731 currently)
    axis_nbounds = 2 ;
variables:
    double bounds_nav_lon(y, x, nvertex) ;
    double bounds_nav_lat(y, x, nvertex) ;
    double time(time) ;
        time:axis = "T" ;
        time:standard_name = "time" ;
        time:long_name = "Time axis" ;
        time:calendar = "gregorian" ;
        time:units = "days since 1949-01-01 00:00:00" ;
        time:time_origin = "1949-01-01 00:00:00" ;
        time:bounds = "time_bounds" ;
    double time_bounds(time, axis_nbounds) ;
    float snw(time, y, x) ;
        snw:standard_name = "surface_snow_amount" ;
        snw:long_name = "Surface Snow Amount" ;
        snw:units = "kg m-2" ;
        snw:online_operation = "average" ;
        snw:interval_operation = "450 s" ;
        snw:interval_write = "1 d" ;
        snw:cell_methods = "time: mean" ;
        snw:_FillValue = 1.e+20f ;
        snw:missing_value = 1.e+20f ;
        snw:coordinates = "lon lat" ;
        snw:grid_mapping = "Lambert_Conformal" ;
    double lon(y, x) ;
        lon:standard_name = "longitude" ;
        lon:long_name = "Longitude" ;
        lon:units = "degrees_east" ;
        lon:bounds = "bounds_nav_lon" ;
    double lat(y, x) ;
        lat:standard_name = "latitude" ;
        lat:long_name = "Latitude" ;
        lat:units = "degrees_north" ;
        lat:bounds = "bounds_nav_lat" ;
    char Lambert_Conformal ;
        Lambert_Conformal:latitude_of_projection_origin = 49.5f ;
        Lambert_Conformal:standard_parallel = 49.5f ;
        Lambert_Conformal:longitude_of_central_meridian = 10.5f ;
        Lambert_Conformal:grid_mapping_name = "lambert_conformal_conic" ;
    double x(x) ;
        x:units = "km" ;
        x:long_name = "x coordinate of projection" ;
        x:standard_name = "projection_x_coordinate" ;
        x:axis = "X" ;
    double y(y) ;
        y:units = "km" ;
        y:long_name = "y coordinate of projection" ;
        y:standard_name = "projection_y_coordinate" ;
        y:axis = "Y" ;

// global attributes:
        :description = "Created by xios" ;
        :title = "Created by xios" ;
        :Conventions = "CF-1.6" ;
        :creation_date = "2018-09-28T16:13:44Z" ;
        :name = "EUR-11_ECMWF-ERAINT_evaluation_r1i1p1_CNRM-ALADIN63_v1_day" ;
        :institute_id = "CNRM" ;
        :institution = "CNRM (Centre National de Recherches Meteorologiques, Toulouse 31057, France)" ;
        :model_id = "CNRM-ALADIN63" ;
        :experiment_id = "evaluation" ;
        :experiment = "Evaluation run with reanalysis forcing" ;
        :contact = "contact.aladin-cordex@meteo.fr" ;
        :product = "output" ;
        :Convention = "CF-1.6" ;
        :driving_model_id = "ECMWF-ERAINT" ;
        :driving_model_ensemble_member = "r1i1p1" ;
        :driving_experiment_name = "evaluation" ;
        :driving_experiment = "ERA-INTERIM, evaluation, r1i1p1" ;
        :rcm_version_id = "v1" ;
        :project_id = "CORDEX" ;
        :CORDEX_domain = "EUR-11" ;
        :references = "http://www.umr-cnrm.fr/spip.php?article125&lang=en" ;
        :comment = "CORDEX Europe EUR-11 CNRM-ALADIN 6.3 L91 ECMWF-ERAINT: EUC12v63-1.100. Reference : Daniel M., Lemonsu A., Déqué M., Somot S., Alias A., Masson V. (2018) Benefits of explicit urban parametrization in regional climate modelling to study climate and city interactions. Climate Dynamics, 1-20, doi:10.1007/s00382-018-4289-x" ;
        :frequency = "day" ;
        :tracking_id = "hdl:21.14103/0efca508-2ffb-420b-8093-fffe58a7ae75" ;
}
cf. #3629 ping @zklaus
SciTools / iris

false_easting = None: loading OK, but saving gives an error #3675
