In [1]: import xarray as xr
In [2]: import numpy as np
In [3]: xr.DataArray(np.random.rand(2, 3, 4), coords=[np.arange(2), np.arange(3), np.arange(4)], dims=['x', 'y', 'z']).to_dataset(name='test').to_netcdf('test_mfdataset.nc')
In [4]: xr.open_dataset('test_mfdataset.nc').test.argmin('x').values
Out[4]:
array([[1, 1, 1, 1],
       [1, 0, 1, 0],
       [1, 1, 0, 1]])

In [5]: xr.open_mfdataset('test_mfdataset.nc').test.argmin('x').values
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-8-ccac9ca40874> in <module>()
----> 1 xr.open_mfdataset('test_mfdataset.nc').test.argmin('x').values

//anaconda/lib/python2.7/site-packages/xarray/core/dataarray.py in values(self)
    353     def values(self):
    354         """The array's data as a numpy.ndarray"""
--> 355         return self.variable.values
    356
    357     @values.setter

//anaconda/lib/python2.7/site-packages/xarray/core/variable.py in values(self)
    286     def values(self):
    287         """The variable's data as a numpy.ndarray"""
--> 288         return _as_array_or_item(self._data_cached())
    289
    290     @values.setter

//anaconda/lib/python2.7/site-packages/xarray/core/variable.py in _data_cached(self)
    252     def _data_cached(self):
    253         if not isinstance(self._data, (np.ndarray, PandasIndexAdapter)):
--> 254             self._data = np.asarray(self._data)
    255         return self._data
    256

//anaconda/lib/python2.7/site-packages/numpy/core/numeric.pyc in asarray(a, dtype, order)
    472
    473     """
--> 474     return array(a, dtype, copy=False, order=order)
    475
    476 def asanyarray(a, dtype=None, order=None):

//anaconda/lib/python2.7/site-packages/dask/array/core.py in __array__(self, dtype, **kwargs)
    852
    853     def __array__(self, dtype=None, **kwargs):
--> 854         x = self.compute()
    855         if dtype and x.dtype != dtype:
    856             x = x.astype(dtype)

//anaconda/lib/python2.7/site-packages/dask/base.py in compute(self, **kwargs)
     35
     36     def compute(self, **kwargs):
---> 37         return compute(self, **kwargs)[0]
     38
     39     @classmethod

//anaconda/lib/python2.7/site-packages/dask/base.py in compute(*args, **kwargs)
    108                 for opt, val in groups.items()])
    109     keys = [var._keys() for var in variables]
--> 110     results = get(dsk, keys, **kwargs)
    111
    112     results_iter = iter(results)

//anaconda/lib/python2.7/site-packages/dask/threaded.py in get(dsk, result, cache, num_workers, **kwargs)
     55     results = get_async(pool.apply_async, len(pool._pool), dsk, result,
     56                         cache=cache, queue=queue, get_id=_thread_get_id,
---> 57                         **kwargs)
     58
     59     return results

//anaconda/lib/python2.7/site-packages/dask/async.py in get_async(apply_async, num_workers, dsk, result, cache, queue, get_id, raise_on_exception, rerun_exceptions_locally, callbacks, **kwargs)
    479                 _execute_task(task, data)  # Re-execute locally
    480             else:
--> 481                 raise(remote_exception(res, tb))
    482         state['cache'][key] = res
    483         finish_task(dsk, key, state, results, keyorder.get)

IndexError: shape mismatch: indexing arrays could not be broadcast together with shapes (3,4) (4,1) (1,3)

Traceback
---------
  File "//anaconda/lib/python2.7/site-packages/dask/async.py", line 264, in execute_task
    result = _execute_task(task, data)
  File "//anaconda/lib/python2.7/site-packages/dask/async.py", line 246, in _execute_task
    return func(*args2)
  File "//anaconda/lib/python2.7/site-packages/toolz/functoolz.py", line 381, in __call__
    ret = f(ret)
  File "//anaconda/lib/python2.7/site-packages/dask/array/reductions.py", line 450, in arg_agg
    return _arg_combine(data, axis, argfunc)[0]
  File "//anaconda/lib/python2.7/site-packages/dask/array/reductions.py", line 416, in _arg_combine
    arg = (arg + offsets)[tuple(inds)]