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
717217552,MDU6SXNzdWU3MTcyMTc1NTI=,4495,`combine_by_coords` fails giving StopIteration error,20053498,closed,0,,,5,2020-10-08T10:24:42Z,2020-10-13T07:03:24Z,2020-10-13T07:03:24Z,NONE,,,,"The dataset is WFDEI historical tasmax downloaded from ESGF. (isimip2a.input.clim_atm_obs.WFDEI.historical.tasmax.daily.v20190812|esg.pik-potsdam.de) Can provide a wget script if necessary.

The code is as follows:
```
import xarray as xr
from pathlib import Path
directory = Path(""/gws/nopw/j04/bas_climate/users/champs/WFDEI"")
input_paths = list(directory.glob('tasmax_*nc'))
xr.open_mfdataset(input_paths, combine='by_coords', use_cftime=True)
```

The error message
```
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-2-5f457963cf48> in <module>
      4 directory = Path(""/gws/nopw/j04/bas_climate/users/champs/WFDEI"")
      5 input_paths = list(directory.glob('tasmax_*nc'))
----> 6 xr.open_mfdataset(input_paths, combine='by_coords', use_cftime=True)

~/miniconda3/envs/ucd/lib/python3.7/site-packages/xarray/backends/api.py in open_mfdataset(paths, chunks, concat_dim, compat, preprocess, engine, lock, data_vars, coords, combine, autoclose, parallel, join, attrs_file, **kwargs)
    957             # previously
    958             combined = combine_by_coords(
--> 959                 datasets, compat=compat, data_vars=data_vars, coords=coords, join=join
    960             )
    961         else:

~/miniconda3/envs/ucd/lib/python3.7/site-packages/xarray/core/combine.py in combine_by_coords(datasets, compat, data_vars, coords, fill_value, join)
    700             compat=compat,
    701             fill_value=fill_value,
--> 702             join=join,
    703         )
    704

~/miniconda3/envs/ucd/lib/python3.7/site-packages/xarray/core/combine.py in _combine_nd(combined_ids, concat_dims, data_vars, coords, compat, fill_value, join)
    180     """"""
    181
--> 182     example_tile_id = next(iter(combined_ids.keys()))
    183
    184     n_dims = len(example_tile_id)

StopIteration:
```

Each of the files can be opened individually with `xr.open_dataset`.

**Environment**:

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.7 (default, Mar 23 2020, 22:36:06)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.10.0-1127.19.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.1

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.3.2
netCDF4: 1.4.2
pydap: None
h5netcdf: 0.8.0
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.1.2
nc_time_axis: 1.2.0
PseudoNetCDF: None
rasterio: 1.0.21
cfgrib: None
iris: 2.4.0
bottleneck: None
dask: 2.11.0
distributed: 2.11.0
matplotlib: 3.1.3
cartopy: 0.17.0
seaborn: None
numbagg: None
setuptools: 46.2.0.post20200511
pip: 20.0.2
conda: None
pytest: None
IPython: 7.13.0
sphinx: None

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