Error -1000 from clEnqueueCopyImageToBuffer

I’m using openGL and openCL on a Titan X / Ubuntu 14.04, with NVidia drivers v352. I have a number of legacy GL shaders that leave a result in an FBO. I’m trying to copy that data into a 1D openCL buffer with clEnqueueCopyImageToBuffer. I’m getting error code = -1000! (Not a listed CL error code, afaik).

Anybody has a different experience?

is this only for this particular opencl primitive?
does any other opencl primitive work?