Aperture 3.5.1 says this library was opened with a newer version of Aperture, repairing it with this version may damage the library
I launched Aperture with Option Command on opening so that I could repair the unified database. After I select repair from the menu, I keep getting a strange error message that says "This library has been opened with a newer version of Aperture. Repairing the library using this version of Aperture may remove data supported only in newer versions of Aperture." I selected cancel and checked that I am running Aperture 3.5.1 and there are no updates showing up when I select Software Update. So how could I have opened it with a newer version of Aperture before? And what is it going to remove?
Aperture is telling me that my unified library needs to be repaired. I figured that out myself when photos started disappearing in events and albums in iPhoto, even though the event or album showed several photos were there in the number of photos in the top right, no images appeared. But I have never received this error message in Aperture before and I do not know where it is coming from or what it is going to do.
iMac, OS X Mavericks (10.9), 3.4 GHz Intel Quad-Core i7 mid-2011