id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
290572700,MDU6SXNzdWUyOTA1NzI3MDA=,1849,passing unlimited_dims to to_netcdf triggers  RuntimeError: NetCDF: Invalid argument,500246,closed,0,,,12,2018-01-22T18:43:23Z,2019-06-04T20:41:50Z,2019-06-04T20:41:50Z,CONTRIBUTOR,,,,"For some datafiles with properties I cannot quite reproduce, `.to_netcdf` leads to a `RuntimeError: NetCDF: Invalid argument` if and only if I pass an `unlimited_dims` corresponding to `y`.  The problem is hard to reproduce.  It happens to this particular dataset, but not to seemingly identical ones created from scratch.  I attach `sample.nc` (gzipped so github would let me upload it).

```
$ cat mwe.py 
#!/usr/bin/env python3.6
import xarray

ds = xarray.open_dataset(""sample.nc"")
ds.to_netcdf(""sample2.nc"", unlimited_dims=[""y""])
$ ncdump sample.nc 
netcdf sample {
dimensions:
        y = 6 ;
variables:
        float x(y) ;
                x:_FillValue = NaNf ;
        int64 y(y) ;
data:

 x = 0, 0, 0, 0, 0, 0 ;

 y = 0, 1, 2, 3, 4, 5 ;
}
$ ./mwe.py 
Traceback (most recent call last):
  File ""./mwe.py"", line 5, in <module>
    ds.to_netcdf(""sample2.nc"", unlimited_dims=[""y""])
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/core/dataset.py"", line 1133, in to_netcdf
    unlimited_dims=unlimited_dims)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/api.py"", line 627, in to_netcdf
    unlimited_dims=unlimited_dims)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/core/dataset.py"", line 1070, in dump_to_store
    unlimited_dims=unlimited_dims)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/common.py"", line 254, in store
    *args, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/common.py"", line 221, in store
    unlimited_dims=unlimited_dims)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/netCDF4_.py"", line 339, in set_variables
    super(NetCDF4DataStore, self).set_variables(*args, **kwargs)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/common.py"", line 233, in set_variables
    name, v, check, unlimited_dims=unlimited_dims)
  File ""/dev/shm/gerrit/venv/stable-3.6/lib/python3.6/site-packages/xarray/backends/netCDF4_.py"", line 385, 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


```

#### Output of ``xr.show_versions()``

<details>
# Paste the output here xr.show_versions() here
$ ./mwe.py 

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-696.6.3.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

xarray: 0.10.0+dev39.ge31cf43
pandas: 0.22.0
numpy: 1.14.0
scipy: 1.0.0
netCDF4: 1.3.1
h5netcdf: None
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.16.1
distributed: None
matplotlib: 2.1.2
cartopy: 0.15.1
seaborn: 0.8.1
setuptools: 38.4.0
pip: 9.0.1
conda: 4.3.16
pytest: 3.1.2
IPython: 6.1.0
sphinx: 1.6.2

[sample.nc.gz](https://github.com/pydata/xarray/files/1653178/sample.nc.gz)

</details>
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