"slice bounds out of range" in cache prune

[bep]

panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/spf13/afero.(*UnionFile).Readdir(0xc0004cc640, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/brew/Library/Caches/Homebrew/go_cache/pkg/mod/github.com/spf13/afero@v1.2.0/unionFile.go:192 +0x516
github.com/spf13/afero.(*UnionFile).Readdirnames(0xc0004cc640, 0x1, 0x402b8d2, 0xc0000dc800, 0xc0006677c0, 0x18, 0xc0006677b8)
	/Users/brew/Library/Caches/Homebrew/go_cache/pkg/mod/github.com/spf13/afero@v1.2.0/unionFile.go:196 +0x4d
github.com/gohugoio/hugo/cache/filecache.Caches.Prune.func1(0x4ca8565, 0x1, 0x4ef2820, 0xc00000d3a0, 0x0, 0x0, 0x0, 0x0)
	/private/tmp/hugo-20181224-98242-12trytm/hugo-0.53/src/github.com/gohugoio/hugo/cache/filecache/filecache_pruner.go:45 +0x12b
github.com/spf13/afero.walk(0x4efada0, 0xc00000c500, 0x0, 0x0, 0x4ef2820, 0xc00000d3a0, 0xc00000d360, 0x0, 0xc00000d360)
	/Users/brew/Library/Caches/Homebrew/go_cache/pkg/mod/github.com/spf13/afero@v1.2.0/path.go:44 +0x8a
github.com/spf13/afero.Walk(0x4efada0, 0xc00000c500, 0x0, 0x0, 0xc00000d360, 0x0, 0x0)
	/Users/brew/Library/Caches/Homebrew/go_cache/pkg/mod/github.com/spf13/afero@v1.2.0/path.go:105 +0xe5
github.com/gohugoio/hugo/cache/filecache.Caches.Prune(0xc0001e4360, 0xc0004568f0, 0x48b57e1, 0xc00056ba48)
	/private/tmp/hugo-20181224-98242-12trytm/hugo-0.53/src/github.com/gohugoio/hugo/cache/filecache/filecache_pruner.go:31 +0x167
github.com/gohugoio/hugo/hugolib.(*HugoSites).GC(0xc0000b3640, 0xc000069260, 0x4ca88d0, 0x2)
	/private/tmp/hugo-20181224-98242-12trytm/hugo-0.53/src/github.com/gohugoio/hugo/hugolib/prune_resources.go:18 +0x33
github.com/gohugoio/hugo/commands.(*commandeer).fullBuild(0xc00024e240, 0x0, 0x0)
	/private/tmp/hugo-20181224-98242-12trytm/hugo-0.53/src/github.com/gohugoio/hugo/commands/hugo.go:333 +0x26c
github.co