diff --git a/pkg/cmd/repo_index.go b/pkg/cmd/repo_index.go
index ece0ce811..095d9a4d7 100644
--- a/pkg/cmd/repo_index.go
+++ b/pkg/cmd/repo_index.go
@@ -101,7 +101,9 @@ func index(dir, url, mergeTo string, json bool) error {
 		var i2 *repo.IndexFile
 		if _, err := os.Stat(mergeTo); errors.Is(err, fs.ErrNotExist) {
 			i2 = repo.NewIndexFile()
-			writeIndexFile(i2, mergeTo, json)
+			if err := writeIndexFile(i2, mergeTo, json); err != nil {
+				return fmt.Errorf("merge failed: %w", err)
+			}
 		} else {
 			i2, err = repo.LoadIndexFile(mergeTo)
 			if err != nil {
diff --git a/pkg/cmd/repo_index_test.go b/pkg/cmd/repo_index_test.go
index 67984f3b7..3a31cf7a4 100644
--- a/pkg/cmd/repo_index_test.go
+++ b/pkg/cmd/repo_index_test.go
@@ -22,6 +22,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -141,3 +142,28 @@ func TestRepoIndexFileCompletion(t *testing.T) {
 	checkFileCompletion(t, "repo index", true)
 	checkFileCompletion(t, "repo index mydir", false)
 }
+
+func TestRepoIndexCmdMergeWriteError(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("directory permissions are not enforced on Windows")
+	}
+	if os.Geteuid() == 0 {
+		t.Skip("root bypasses directory permissions")
+	}
+
+	dir := t.TempDir()
+	require.NoError(t, linkOrCopy("testdata/testcharts/compressedchart-0.1.0.tgz", filepath.Join(dir, "compressedchart-0.1.0.tgz")))
+
+	// The merge target does not exist and its directory is read-only, so
+	// creating it must fail and the error must be reported.
+	roDir := filepath.Join(t.TempDir(), "ro")
+	require.NoError(t, os.Mkdir(roDir, 0o500))
+	mergeTo := filepath.Join(roDir, "index.yaml")
+
+	buf := bytes.NewBuffer(nil)
+	c := newRepoIndexCmd(buf)
+	require.NoError(t, c.ParseFlags([]string{"--merge", mergeTo}))
+
+	err := c.RunE(c, []string{dir})
+	require.Error(t, err, "expected an error when the missing merge target cannot be written")
+}