Open Channels on Hog 4...Please Help

I have channels 1-86 patched as a desk channel and then 88 - 312 as a desk channel. Somehow Channel 87 is empty and is showing as 1 Free Channel. I would like to change this available channel to a Desk Channel. Everytime I select Fixture > Desk Channel > @ > Select Universe 1 > @87, it tells me The specified Range Does Not Contain Valid Fixtures. What am I doing wrong. Thanks!