about single point crossover??

i'm writing aprog, convert word to numbers then binary then want to make crossover with random chromosome,the problem face me when the word is 3 letters output 4..and when 2 letters return error..can anyone help me plz

%convert the word 'THIS' to numbers according to the alphabetic sequence

prompt = 'Enter your text: ';

text= input(prompt,'s');

alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ';

[~,pos] = ismember(text,alphabet)

%convert each letter of the word to binary value

step3=de2bi(pos,'left-msb')

n = size(step3, 1);

l = size(step3, 2);

%generate the default chromosome (second parent)

Step4= randi([0, 1], 1,l)

Step4=[Step4;Step4;Step4;Step4];

%Choose a crossover point

cp = randperm(l-1, 1)

for i=1:n

breeders = [step3(i,:);Step4];

b1(i,:) = [breeders(1, 1:cp), breeders(2, cp+1:end)];

b2(i,:) = [breeders(2, 1:cp), breeders(1, cp+1:end)];

i=i+1;

end

b1=b1

b2=b2