Form keep asking to fill in form which is NOT required

I have a field defined in rules() as follows:

array('image', 'file', 'types'=>'jpg, gif, png')),

If I do not specify a file to upload, my application keep asking me to fill it in (Field ‘image’ cannot be empty). I have checked what does rules() return and it does not specify ‘image’ in ‘required’.

Do you what could be the reason? Or could you give me an advice of how to check what is going wrong?

See the allowEmpty property.