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Got it, thanks @shoyer ! In case this happens again, which component of the traceback provided that information to you?

``` TypeErrorTraceback (most recent call last) <ipython-input-152-52564f498ac3> in <module>() ----> 1 ds = xr.open_mfdataset(files[:2], concat_dim='time')

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/backends/api.pyc in open_mfdataset(paths, chunks, concat_dim, preprocess, engine, lock, **kwargs) 304 datasets = [preprocess(ds) for ds in datasets] 305 --> 306 combined = auto_combine(datasets, concat_dim=concat_dim) 307 combined._file_obj = _MultiFileCloser(file_objs) 308 return combined

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/combine.pyc in auto_combine(datasets, concat_dim) 376 grouped = itertoolz.groupby(lambda ds: tuple(sorted(ds.data_vars)), 377 datasets).values() --> 378 concatenated = [_auto_concat(ds, dim=concat_dim) for ds in grouped] 379 merged = merge(concatenated) 380 return merged

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/combine.pyc in _auto_concat(datasets, dim) 327 'explicitly') 328 dim, = concat_dims --> 329 return concat(datasets, dim=dim) 330 331

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/combine.pyc in concat(objs, dim, data_vars, coords, compat, positions, indexers, mode, concat_over) 115 raise TypeError('can only concatenate xarray Dataset and DataArray ' 116 'objects, got %s' % type(first_obj)) --> 117 return f(objs, dim, data_vars, coords, compat, positions) 118 119

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/combine.pyc in _dataset_concat(datasets, dim, data_vars, coords, compat, positions) 206 dim, coord = _calc_concat_dim_coord(dim) 207 datasets = [as_dataset(ds) for ds in datasets] --> 208 datasets = align(*datasets, join='outer', copy=False, exclude=[dim]) 209 210 concat_over = _calc_concat_over(datasets, dim, data_vars, coords)

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/alignment.pyc in align(objects, *kwargs) 78 all_indexes = defaultdict(list) 79 for obj in objects: ---> 80 for dim, index in iteritems(obj.indexes): 81 if dim not in exclude: 82 all_indexes[dim].append(index)

/glade/apps/opt/python/2.7.7/gnu-westmere/4.8.2/lib/python2.7/_abcoll.pyc in iteritems(self) 385 'D.iteritems() -> an iterator over the (key, value) items of D' 386 for key in self: --> 387 yield (key, self[key]) 388 389 def keys(self):

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/coordinates.pyc in getitem(self, key) 245 def getitem(self, key): 246 if key in self: --> 247 return self._variables[key].to_index() 248 else: 249 raise KeyError(key)

/glade/u/home/mckinnon/.local/lib/python2.7/site-packages/xarray/core/variable.pyc in to_index(self) 1173 index = index.set_names(valid_level_names) 1174 else: -> 1175 index = index.set_names(self.name) 1176 return index 1177

/glade/apps/opt/pandas/0.14.0/gnu/4.8.2/lib/python2.7/site-packages/pandas-0.14.0-py2.7-linux-x86_64.egg/pandas/core/index.pyc in set_names(self, names, inplace) 380 """ 381 if not com.is_list_like(names): --> 382 raise TypeError("Must pass list-like as names.") 383 if inplace: 384 idx = self

TypeError: Must pass list-like as names. ```

I'm running into the same problem as Ryan, regarding the ValueError. However, when I try the same fix

ds = xr.open_mfdataset(files[:4], concat_dim = 'time')

I get the error

TypeError: Must pass list-like asnames.

Apologies if this should be on a different chain, but any idea what might be going on?