Trouble with parfor due to the way of indexing a cell

I tried to write the constructor for the Class Analysis I defined. One of its property e is a cell and each element of the cell is a instance of another class Ephys. In the constructor, I wrote a nested for-loop to instantiate each element of e. To accelerate, I used parfor for the inner for-loop but the editor reported that the PARFOR Loop cannot run due to the way variable 'a' is used.

I can't figure out the reason why parfor doen't work, because each element of a.e seems to be independently accessed. Any ideas? Or any other ways to accelerate？Thank you so much!

classdef Analysis

    properties
        e
    end
    
    methods
        
        function a = Analysis(path_txt, path_plx, folder_wav)

            neurons = Spike.split(path_txt);
            songs = Sound.split(folder_wav);
            t = Trigger(path_plx);
            
            for m = 1: length(neurons) % nested for loop, layer 1
                sp = Spike(neurons{m});
                parfor n = 1: length(songs)  %layer 2, where I used parfor
                    so = Sound(songs{n});
                    a.e{m,n} = Ephys(sp,t,so);
                end
            end
        end
        
    end
    
end

[George Bashkatov]

try to change a.e{m,n} to a{m,n}.e