Tristate LVDS

Hello.

Am on the Cyclone III and would like to know what the best way to tri-state or otherwise disable an LVDS output port is.

If I add a tri state buffer between the output pin I recieve an error.

If I try and instantiate ALT_OUTBUF_TRI_DIFF and set the I/O standard to a differential standard (I can only select LVDS or Bus LVDS) the assignment editor automatically, in the words of the fitter, "splits this differntial buffer into a psuedo-differential atom" creating the negative for each of the LVDS output pins.