ebooks and voiceover

The voiceover funtion to read ebooks works really well, but when I lock the screen voiceover tells me "screen locked" and the reading stops. This is really anoying when i go out for a walk or in a bus or subway becouse I cant lock the screen and cant put my iphone on my pocket.

Is there a way to block the screen without block the voiceover reading of the book or article?