screensaver source file keeps changing back to nature photos

I'm using a 2010 MacBook Pro and OS X Yosemite. I set the source file for the screensaver in Preferences to a certain folder of personal photos. After I re-boot, the source file switches back to a default source file . . . in my case, it's the Apple "Nature" Photos. I can't figure out what is causing this change in my screensaver settings.