Always add the word "Subject" when reply e-mail

Hello!

Zoho mail is working great! but I have one thing that bothers me.

When I reply to exists one, zoho mail adds the word "Subject" to the exists Subject.

Example:
I get an email with subject: "Hi, how are you".

When I click reply zoho mail changes my subject like this: "Subject: Hi, how are you".

Why is doing this? Can I change that? I think its a bug, because it should 
never change my subject automatically.

Thanks!