diff --git a/testdata/ffmpeg.d/cmd/prune/main.go b/testdata/ffmpeg.d/cmd/prune/main.go
index 0516835..2e1883d 100644
--- a/testdata/ffmpeg.d/cmd/prune/main.go
+++ b/testdata/ffmpeg.d/cmd/prune/main.go
@@ -51,12 +51,12 @@ func Run(ctx context.Context, args []string) error {
 			series = append(series, strings.Split(path.Base(f), ".")[0])
 		}
 		series = slices.Compact(series)
+		sort.Strings(series)
 		if len(series) < 1 {
 			continue
 		}
-		series = series[:len(series)-1]
 
-		for _, series := range series {
+		for _, series := range series[:len(series)-1] {
 			if err := func() error {
 				seriesFiles := []string{}
 				for _, file := range files {