Further modifies the _add_ method.

ndcube/ndcube.py

@@ -1044,10 +1044,21 @@ def __add__(self, value):
             else:
                 raise TypeError("Cannot add unitless NDData to a unitful NDCube.")
 
+            # combine mask
+            self_ma = np.ma.MaskedArray(self.data, mask=self.mask)
+            value_ma = np.ma.MaskedArray(value_data, mask=value.mask)
+
+            # addition
+            result_ma = self_ma + value_ma
+
+            # extract new mask and new data
+            kwargs["mask"] = result_ma.mask
+            kwargs["data"] = result_ma.data
+
             # combine the uncertainty
             if self.uncertainty is not None and value.uncertainty is not None:
                 new_uncertainty = self.uncertainty.propagate(
-                    np.add, value.uncertainty, result_data = value.data, correlation=0
+                    np.add, value.uncertainty, result_data = kwargs["data"], correlation=0
                 )
                 kwargs["uncertainty"] = new_uncertainty
             elif self.uncertainty is not None:
@@ -1058,37 +1069,25 @@ def __add__(self, value):
             else:
                 new_uncertainty = None
 
-            # combine mask
-            self_ma = np.ma.MaskedArray(self.data, mask=self.mask)
-            value_ma = np.ma.MaskedArray(value_data, mask=value.mask)
-
-            # addition
-            result_ma = self_ma + value_ma
-
-            # extract new mask and new data
-            kwargs["mask"] = result_ma.mask
-            kwargs["data"] = result_ma.data
-
-            # return the new NDCube instance
-            return self._new_instance(**kwargs)
-
         if hasattr(value, 'unit'):
             if isinstance(value, u.Quantity):
                 # NOTE: if the cube does not have units, we cannot
                 # perform arithmetic between a unitful quantity.
                 # This forces a conversion to a dimensionless quantity
                 # so that an error is thrown if value is not dimensionless
                 cube_unit = u.Unit('') if self.unit is None else self.unit
-                new_data = self.data + value.to_value(cube_unit)
+                kwargs["data"] = self.data + value.to_value(cube_unit)
             else:
                 # NOTE: This explicitly excludes other NDCube objects and NDData objects
                 # which could carry a different WCS than the NDCube
                 return NotImplemented
         elif self.unit not in (None, u.Unit("")):
             raise TypeError("Cannot add a unitless object to an NDCube with a unit.")
         else:
-            new_data = self.data + value
-        return self._new_instance(data=new_data)
+            kwargs["data"] = self.data + value
+
+        # return the new NDCube instance
+        return self._new_instance(**kwargs)
 
     def __radd__(self, value):
         return self.__add__(value)