Matlab keep give me following error message :

??? Error using ==> dir Argument must contain a string.

Error in ==> Awal at 15 x = dir(subDirs)

Below is my codes :

%MY PROGRAM

clear all;
clc;
close all;

%-----Create Database-----
TrainDB = uigetdir('','Select Database Directory');
TrainFiles = dir(TrainDB);
dirIndex = [TrainFiles.isdir];
[s subDirNumber] = size(dirIndex);
for i = 3:subDirNumber
    subDirs = {TrainFiles(i).name};
    subDirs = strcat(TrainDB,'\',subDirs);
    x = dir(subDirs) %<-------Error Here
end

Is something wrong with the codes? Your help will be appreciated. I'm sorry for my bad English.