diff --git a/cmd/restic/cmd_forget.go b/cmd/restic/cmd_forget.go
index c00ff1dd5..d2ddd5386 100644
--- a/cmd/restic/cmd_forget.go
+++ b/cmd/restic/cmd_forget.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"strconv"
+	"sync"
 
 	"github.com/restic/restic/internal/data"
 	"github.com/restic/restic/internal/errors"
@@ -317,10 +318,13 @@ func runForget(ctx context.Context, opts ForgetOptions, pruneOptions PruneOption
 	if len(removeSnIDs) > 0 {
 		if !opts.DryRun {
 			bar := printer.NewCounter("files deleted")
+			var lock sync.Mutex
 			err := restic.ParallelRemove(ctx, repo, removeSnIDs, restic.WriteableSnapshotFile, func(id restic.ID, err error) error {
 				if err != nil {
 					printer.E("unable to remove %v/%v from the repository\n", restic.SnapshotFile, id)
+					lock.Lock()
 					failedSnIDs.Insert(id)
+					lock.Unlock()
 				} else {
 					printer.VV("removed %v/%v\n", restic.SnapshotFile, id)
 				}