How to remove the transparent field around the plank dock?

I have installed plank and noticed the transparent field with a distinct border (which is not only visible, windows even stick to it when dragged near) around it. Below a picture where I have highlighted a part of it. How to get rid of this effect? I have seen many screen shots of plank without this but switching themes in the settings doesn't seem to help. Nor could I find the option to disable it.