Using glReadPixels to read from ofFBO?

I’m trying to read a single pixel from my FBO with glReadPixels, but I’m getting an invalid operation error when checking the OpenGL error.
I think I have to do something along the lines of

glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, fbo.getTexture().getTextureData().textureID);

before I call glReadPixels. But I think i’m binding the wrong thing, it too is giving an invalid operation error.

The FBO is bound at that point trough ofFbo::begin().