abs and sqrt on image return an error

In [1]: from mpdaf.obj import Cube

In [2]: cube = Cube('data/sdetect/minicube.fits')

In [3]: ima = cube.sum(axis=0)

In [4]: ima.sqrt()

ValueError Traceback (most recent call last) in () ----> 1 ima.sqrt()

/home/piqueras/.local/lib/python2.7/site-packages/mpdaf-1.3.dev1762-py2.7-linux-x86_64.egg/mpdaf/obj/data.pyc in sqrt(self, out) 973 out = self.clone() 974 --> 975 out.data = np.ma.sqrt(self.data) 976 out.unit = self.unit / u.Unit(np.sqrt(self.unit.scale)) 977

/home/piqueras/.local/lib/python2.7/site-packages/mpdaf-1.3.dev1762-py2.7-linux-x86_64.egg/mpdaf/obj/data.pyc in data(self, value) 482 if self.shape is not None and
483 not np.array_equal(value.shape, self.shape): --> 484 raise ValueError('try to set data with an array with a different ' 485 'shape') 486

ValueError: try to set data with an array with a different shape

The same error is returned by ima.abs()