html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/1849#issuecomment-369543072,https://api.github.com/repos/pydata/xarray/issues/1849,369543072,MDEyOklzc3VlQ29tbWVudDM2OTU0MzA3Mg==,8441217,2018-03-01T10:09:40Z,2018-03-01T10:09:40Z,CONTRIBUTOR,"I also ran into this issue : to_netcdf fails for my dataset.

Here is how to reproduce the error : (the testfile is attached here [1.zip](https://github.com/pydata/xarray/files/1770670/1.zip) )

```
import netCDF4                                                                                                                                                                           
import xarray as xr                                                                                                                                                                      
print(netCDF4.__version__)                                                                                                                                                               
print(xr.__version__)                                                                                                                                                                    
ds = xr.open_dataset('testfile')                                                                                                                                                         
ds.to_netcdf('outfile.ok')                                                                                                                                                               
ds.to_netcdf('outfile.not.ok', unlimited_dims=['datetime'])  
```

And the ouput I get : 

> 1.3.1
> 0.10.1
> Traceback (most recent call last):
>   File ""bug.py"", line 6, in <module>
>     ds.to_netcdf('outfile', unlimited_dims=['datetime'])
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/core/dataset.py"", line 1133, in to_netcdf
>     unlimited_dims=unlimited_dims)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/backends/api.py"", line 632, in to_netcdf
>     unlimited_dims=unlimited_dims)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/core/dataset.py"", line 1070, in dump_to_store
>     unlimited_dims=unlimited_dims)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/backends/common.py"", line 280, in store
>     unlimited_dims=unlimited_dims)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/backends/netCDF4_.py"", line 348, in set_variables
>     super(NetCDF4DataStore, self).set_variables(*args, **kwargs)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/backends/common.py"", line 317, in set_variables
>     name, v, check, unlimited_dims=unlimited_dims)
>   File ""/home/pinaultf/miniconda3/envs/defaultenv/lib/python3.6/site-packages/xarray/backends/netCDF4_.py"", line 393, in prepare_variable
>     fill_value=fill_value)
>   File ""netCDF4/_netCDF4.pyx"", line 2437, in netCDF4._netCDF4.Dataset.createVariable
>   File ""netCDF4/_netCDF4.pyx"", line 3439, in netCDF4._netCDF4.Variable.__init__
>   File ""netCDF4/_netCDF4.pyx"", line 1638, in netCDF4._netCDF4._ensure_nc_success
> RuntimeError: NetCDF: Invalid argument
> 


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