Why do I get an "Invalid value returned."-error when assigning a string property inside a loop using v18b?

When initializing an object of class "Matlab_Dbl_Str_Test" (class in attached file below) in Matlab v18b the result is the following:

When avoiding the try catch block one gets the above mentioned error.

It appears that properties of type string cannot be assigned in a loop whereas properties of type double are perfectly fine.

Is this a known bug or wanted feauture?

%% --------------------------------------------------------------------------
classdef Matlab_Dbl_Str_Test < handle
    %UNTITLED2 Summary of this class goes here
    %   Detailed explanation goes here
    properties( Constant )
        TEST_STR = ["a";"b";"c"];
        TEST_DBL = [0;1;2];        
    end
    properties( SetAccess = protected )
        test_str@string as vector;
        test_dbl@double as vector;
    end
    
    methods
        function obj = Matlab_Dbl_Str_Test()
            for i = 1:3
                try obj.test_str(i) = Matlab_Dbl_Str_Test.TEST_STR(i,1);end
                try obj.test_dbl(i) = Matlab_Dbl_Str_Test.TEST_DBL(i,1);end
            end
        end
        function test( obj )
            for i = 1:3
                try obj.test_str(i) = Matlab_Dbl_Str_Test.TEST_STR(i,1);end
                try obj.test_dbl(i) = Matlab_Dbl_Str_Test.TEST_DBL(i,1);end
            end
        end
    end
end

Answer 1

[per isakson]

The syntax of your properties definitions

    properties( SetAccess = protected )
        test_str@string as vector;
        test_dbl@double as vector;
    end

looks strange to me and I failed to find it in the documentation. (Maybe I didn't try hard enough.)

This modified version of your class works with R2018b

classdef Matlab_Dbl_Str_Test < handle
    properties( Constant )
        TEST_STR = ["a";"b";"c"];
        TEST_DBL = [0;1;2];        
    end
    properties( SetAccess = protected )
        test_str (:,1) string
        test_dbl (:,1) double
    end
    methods
        function obj = Matlab_Dbl_Str_Test()
            for i = 1:3
                try 
                    obj.test_str(i) = Matlab_Dbl_Str_Test.TEST_STR(i,1);
                catch me
                    getReport( me )
                end
                try 
                    obj.test_dbl(i) = Matlab_Dbl_Str_Test.TEST_DBL(i,1);
                catch me
                    getReport( me )
                end
            end
        end
        function test( obj )
            for i = 1:3
                try 
                    obj.test_str(i) = Matlab_Dbl_Str_Test.TEST_STR(i,1);
                catch me
                    getReport( me )
                end
                try 
                    obj.test_dbl(i) = Matlab_Dbl_Str_Test.TEST_DBL(i,1);
                catch me
                    getReport( me )
                end
            end
        end
    end
end

Test

>> mbs = Matlab_Dbl_Str_Test
mbs = 
  Matlab_Dbl_Str_Test with properties:

    TEST_STR: [3×1 string]
    TEST_DBL: [3×1 double]
    test_str: [3×1 string]
    test_dbl: [3×1 double]
>> mbs.test_str
ans = 
  3×1 string array
    "a"
    "b"
    "c"