issues: 335640601

I'm brand new (TODAY) to xarray, so I'm not going to use the right lingo... I want to save a wrf diagnostic variable 'pvo' (https://wrf-python.readthedocs.io/en/latest/diagnostics.html#diagnostic-table) to its own new netcdf file.

```python dataset = xr.Dataset({'pvo':pvo}) dataset.info() Coordinates: XLONG float32 -101.73564 XLAT float32 1.7164688 Time datetime64[ns] 2014-09-15T21:00:00 xarray.Dataset { dimensions: bottom_top = 42 ; south_north = 243 ; west_east = 378 ;

variables: float32 XLONG(south_north, west_east) ; float32 XLAT(south_north, west_east) ; datetime64[ns] Time() ; float32 pvo(bottom_top, south_north, west_east) ; pvo:FieldType = 104 ; pvo:MemoryOrder = XYZ ; pvo:description = potential vorticity ; pvo:units = PVU ; pvo:stagger = ; pvo:coordinates = XLONG XLAT ; pvo:projection = Mercator(stand_lon=-65.0, moad_cen_lat=23.722755432128906, truelat1=30.0, truelat2=60.0, pole_lat=90.0, pole_lon=0.0) ; pvo:_FillValue = 9.969209968386869e+36 ; pvo:missing_value = 9.969209968386869e+36 ;

``` Looks great, let's save it (am I doing this right?)

python pvo.to_dataset().to_netcdf("analysis_d01_2014-09-15_21h.nc","a") It breaks and complains about the projection attribute:

python Traceback (most recent call last): File "matplotwrf.py", line 86, in <module> pvo.to_dataset().to_netcdf("analysis_d01_2014-09-15_21h.nc","a") File "/software/Anaconda3-5.1.0-el6-x86_64/envs/wrf-python/lib/python3.6/site-packages/xarray/core/dataset.py", line 1150, in to_netcdf compute=compute) File "/software/Anaconda3-5.1.0-el6-x86_64/envs/wrf-python/lib/python3.6/site-packages/xarray/backends/api.py", line 659, in to_netcdf _validate_attrs(dataset) File "/software/Anaconda3-5.1.0-el6-x86_64/envs/wrf-python/lib/python3.6/site-packages/xarray/backends/api.py", line 120, in _validate_attrs check_attr(k, v) File "/software/Anaconda3-5.1.0-el6-x86_64/envs/wrf-python/lib/python3.6/site-packages/xarray/backends/api.py", line 111, in check_attr 'files'.format(value)) TypeError: Invalid value for attr: Mercator(stand_lon=-65.0, moad_cen_lat=23.722755432128906, truelat1=30.0, truelat2=60.0, pole_lat=90.0, pole_lon=0.0) must be a number string, ndarray or a list/tuple of numbers/strings for serialization to netCDF files

Do I have to chop up, rework or remove the projection attribute? Surely these diagnostic variables are immediately ready to save as complete netcdf variables, what am I doing wrong?

Thank you!