Refresh variable cycle Matlab

I am currently creating a Matlab script to generate a file with the characteristics of a signal. This is the code used:

clear;
folder = dir('sonidos');
m = length(folder);

fileW = fopen('caracteristicas.txt','wt');

for i = 3:m
    file = strcat('sonidos/', folder(i).name);
    fprintf('Leyendo Archivo: %s \n', file);
    [y, Fs] = audioread(file);
     max = max(y);
    mean = mean(y);
    min = min(y);
    median = median(y);
    peak2peak = peak2peak(y);
    peak2rms = peak2rms(y);
    rms = rms(y);
    rssq = rssq(y);
    std = std(y);
    var = var(y);
    kurtosis = kurtosis(y);
    power = (norm(y)^2)/length(y);
    fprintf('Caracteristicas: %f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f \n', ...
            Fs, max, mean, min, median, peak2peak, peak2rms, rms, rssq, std, var, kurtosis, power);
    fprintf(fileW,'%f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f,%f \n', ...
                  Fs, max, mean, min, median, peak2peak, peak2rms, rms, rssq, std, var, kurtosis, power);
end

fclose(fileW);

What I want is to read the sound files in the 'sounds' directory, get the characteristics of the signal and then write the obtained characteristics in a flat file.

When it enters the first cycle the program works fine, but in the second cycle the following appears:

Subscript indices must either be real positive integers or logicals.

Any ideas to solve the error? Thanks