iOS <Error>: CGContextDraw

<Error>: CGContextDrawImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

<Error>: CGBitmapContextCreateImage: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.

网上查了很多，说是iOS6的bug，不影响程序的运行上线，可是出现这个bug很难受啊