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/3037#issuecomment-509518811,https://api.github.com/repos/pydata/xarray/issues/3037,509518811,MDEyOklzc3VlQ29tbWVudDUwOTUxODgxMQ==,32473508,2019-07-09T07:09:05Z,2019-07-09T07:14:44Z,CONTRIBUTOR,"@shoyer I would like to work on this issue (hopefully my first contribution). 

I believe the error should indicate that we cannot groupby an empty group. This would be consistent with the documentation:

```
DataArray.groupby(group, squeeze: bool = True, restore_coord_dims: Optional[bool] = None)
Parameters
groupstr, DataArray or IndexVariable
Array whose unique values should be used to group this array. If a string, must be the name of a variable contained in this dataset.
````

For the groupby class, we can raise an error in case the groupby object instantiation does not specify values to be grouped by. In the case above:

```
ValueError: variable to groupby must not be empty

```

If you find such a solution acceptable, I would create a pull request.

Thank you in advance","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,459389883