diff --git a/cmd/restic/cmd_prune.go b/cmd/restic/cmd_prune.go
index bdf8951ed..2637fedc8 100644
--- a/cmd/restic/cmd_prune.go
+++ b/cmd/restic/cmd_prune.go
@@ -9,7 +9,6 @@ import (
 
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
-	"github.com/restic/restic/internal/index"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/ui"
@@ -189,8 +188,8 @@ func runPruneWithRepo(ctx context.Context, opts PruneOptions, gopts GlobalOption
 		RepackUncompressed: opts.RepackUncompressed,
 	}
 
-	plan, err := repository.PlanPrune(ctx, popts, repo, func(ctx context.Context, repo restic.Repository) (usedBlobs *index.AssociatedSet[uint8], err error) {
-		return getUsedBlobs(ctx, repo, ignoreSnapshots, printer)
+	plan, err := repository.PlanPrune(ctx, popts, repo, func(ctx context.Context, repo restic.Repository, usedBlobs restic.FindBlobSet) error {
+		return getUsedBlobs(ctx, repo, usedBlobs, ignoreSnapshots, printer)
 	}, printer)
 	if err != nil {
 		return err
@@ -256,10 +255,10 @@ func printPruneStats(printer progress.Printer, stats repository.PruneStats) erro
 	return nil
 }
 
-func getUsedBlobs(ctx context.Context, repo restic.Repository, ignoreSnapshots restic.IDSet, printer progress.Printer) (usedBlobs *index.AssociatedSet[uint8], err error) {
+func getUsedBlobs(ctx context.Context, repo restic.Repository, usedBlobs restic.FindBlobSet, ignoreSnapshots restic.IDSet, printer progress.Printer) error {
 	var snapshotTrees restic.IDs
 	printer.P("loading all snapshots...\n")
-	err = restic.ForAllSnapshots(ctx, repo, repo, ignoreSnapshots,
+	err := restic.ForAllSnapshots(ctx, repo, repo, ignoreSnapshots,
 		func(id restic.ID, sn *restic.Snapshot, err error) error {
 			if err != nil {
 				debug.Log("failed to load snapshot %v (error %v)", id, err)
@@ -270,20 +269,14 @@ func getUsedBlobs(ctx context.Context, repo restic.Repository, ignoreSnapshots r
 			return nil
 		})
 	if err != nil {
-		return nil, errors.Fatalf("failed loading snapshot: %v", err)
+		return errors.Fatalf("failed loading snapshot: %v", err)
 	}
 
 	printer.P("finding data that is still in use for %d snapshots\n", len(snapshotTrees))
 
-	usedBlobs = index.NewAssociatedSet[uint8](repo.Index().(*index.MasterIndex))
-
 	bar := printer.NewCounter("snapshots")
 	bar.SetMax(uint64(len(snapshotTrees)))
 	defer bar.Done()
 
-	err = restic.FindUsedBlobs(ctx, repo, snapshotTrees, usedBlobs, bar)
-	if err != nil {
-		return nil, err
-	}
-	return usedBlobs, nil
+	return restic.FindUsedBlobs(ctx, repo, snapshotTrees, usedBlobs, bar)
 }
diff --git a/internal/repository/prune.go b/internal/repository/prune.go
index f059fc0d0..1247a6b3e 100644
--- a/internal/repository/prune.go
+++ b/internal/repository/prune.go
@@ -91,7 +91,7 @@ type packInfoWithID struct {
 
 // PlanPrune selects which files to rewrite and which to delete and which blobs to keep.
 // Also some summary statistics are returned.
-func PlanPrune(ctx context.Context, opts PruneOptions, repo *Repository, getUsedBlobs func(ctx context.Context, repo restic.Repository) (usedBlobs *index.AssociatedSet[uint8], err error), printer progress.Printer) (*PrunePlan, error) {
+func PlanPrune(ctx context.Context, opts PruneOptions, repo *Repository, getUsedBlobs func(ctx context.Context, repo restic.Repository, usedBlobs restic.FindBlobSet) error, printer progress.Printer) (*PrunePlan, error) {
 	var stats PruneStats
 
 	if opts.UnsafeRecovery {
@@ -105,7 +105,8 @@ func PlanPrune(ctx context.Context, opts PruneOptions, repo *Repository, getUsed
 		return nil, fmt.Errorf("compression requires at least repository format version 2")
 	}
 
-	usedBlobs, err := getUsedBlobs(ctx, repo)
+	usedBlobs := index.NewAssociatedSet[uint8](repo.idx)
+	err := getUsedBlobs(ctx, repo, usedBlobs)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/repository/prune_test.go b/internal/repository/prune_test.go
index 6158e9066..17dfafa4e 100644
--- a/internal/repository/prune_test.go
+++ b/internal/repository/prune_test.go
@@ -6,7 +6,6 @@ import (
 	"testing"
 
 	"github.com/restic/restic/internal/checker"
-	"github.com/restic/restic/internal/index"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 	rtest "github.com/restic/restic/internal/test"
@@ -31,12 +30,11 @@ func testPrune(t *testing.T, opts repository.PruneOptions, errOnUnused bool) {
 	}
 	rtest.OK(t, repo.Flush(context.TODO()))
 
-	plan, err := repository.PlanPrune(context.TODO(), opts, repo, func(ctx context.Context, repo restic.Repository) (usedBlobs *index.AssociatedSet[uint8], err error) {
-		idx := index.NewAssociatedSet[uint8](repo.Index().(*index.MasterIndex))
+	plan, err := repository.PlanPrune(context.TODO(), opts, repo, func(ctx context.Context, repo restic.Repository, usedBlobs restic.FindBlobSet) error {
 		for blob := range keep {
-			idx.Insert(blob)
+			usedBlobs.Insert(blob)
 		}
-		return idx, nil
+		return nil
 	}, &progress.NoopPrinter{})
 	rtest.OK(t, err)
 
diff --git a/internal/restic/find.go b/internal/restic/find.go
index cefef2196..d7b032bf8 100644
--- a/internal/restic/find.go
+++ b/internal/restic/find.go
@@ -15,14 +15,14 @@ type Loader interface {
 	Connections() uint
 }
 
-type findBlobSet interface {
+type FindBlobSet interface {
 	Has(bh BlobHandle) bool
 	Insert(bh BlobHandle)
 }
 
 // FindUsedBlobs traverses the tree ID and adds all seen blobs (trees and data
 // blobs) to the set blobs. Already seen tree blobs will not be visited again.
-func FindUsedBlobs(ctx context.Context, repo Loader, treeIDs IDs, blobs findBlobSet, p *progress.Counter) error {
+func FindUsedBlobs(ctx context.Context, repo Loader, treeIDs IDs, blobs FindBlobSet, p *progress.Counter) error {
 	var lock sync.Mutex
 
 	wg, ctx := errgroup.WithContext(ctx)