ColorAttachment enums on ReadBufferMode?

I tried using FrameBuffers for rendering to a bitmap file. the idea is first rendering to a texture via framebuffer and then reading pixels from framebuffer for IO. It sufficed to set draw buffer to colorattachment_zero and render to framebuffer. But while setting read buffer (GL.ReadBuffer(ReadBufferMode)) Front, left etc. all resulted in InvalidEnum error(using OpenTK in debug mode). however (ReadBufferMode)(int)DrawBufferMode.ColorAttachment0 gave the desired result. Is non-existance of attachments on ReadBufferMode is intended? (So, am i doing something wrong?) Or is it forgotten?