panic: sync: WaitGroup is reused before previous Wait has returned

[rigon]

panic: sync: WaitGroup is reused before previous Wait has returned

goroutine 49 [running]:
sync.(*WaitGroup).Wait(0xe585d0)
        /usr/local/go/src/sync/waitgroup.go:138 +0xc0
main.WaitBackgroundWork(0xb8?)
        /app/worker.go:68 +0x2c
main.AddThumbsBackground(0x40000e7c20, 0x4005ce6070, {0x40087f2000, 0x5023, 0x40005afe38?})
        /app/worker.go:128 +0x8c
main.(*Collection).CreateThumbnails(0x40000e7c20)
        /app/scan.go:104 +0x4dc
main.main.func1()
        /app/main.go:404 +0x154
created by main.main
        /app/main.go:391 +0x2c8