diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
index 4bbd762..75b3df8 100644
--- a/internal/cli/cli_test.go
+++ b/internal/cli/cli_test.go
@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"git.eeqj.de/sneak/keyfunc/internal/cli"
+	"git.eeqj.de/sneak/keyfunc/internal/derive"
 	"git.eeqj.de/sneak/keyfunc/internal/mnemonic"
 	"github.com/stretchr/testify/require"
 	"golang.org/x/crypto/ssh"
@@ -62,6 +63,14 @@ func TestThePrivateKeyMatchesThePublicOne(t *testing.T) {
 	require.Equal(t, vectorOne, back)
 }
 
+func TestAnIndexWithNoHardenedChildIsRefused(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	out, err := execute(t, "ssh", "pub", "-n", "2147483648")
+	require.ErrorIs(t, err, derive.ErrIndexTooLarge)
+	require.Empty(t, out)
+}
+
 func TestTheMnemonicCommandIsUsed(t *testing.T) {
 	t.Setenv(mnemonic.Variable, "")
 
@@ -77,6 +86,17 @@ func TestTheMnemonicCommandIsUsed(t *testing.T) {
 func run(t *testing.T, args ...string) string {
 	t.Helper()
 
+	out, err := execute(t, args...)
+	require.NoError(t, err)
+
+	return out
+}
+
+// execute runs the tool and returns both what it wrote and how it
+// ended.
+func execute(t *testing.T, args ...string) (string, error) {
+	t.Helper()
+
 	var out bytes.Buffer
 
 	root := cli.Root()
@@ -84,7 +104,7 @@ func run(t *testing.T, args ...string) string {
 	root.SetErr(&out)
 	root.SetArgs(args)
 
-	require.NoError(t, root.ExecuteContext(t.Context()))
+	err := root.ExecuteContext(t.Context())
 
-	return out.String()
+	return out.String(), err
 }
diff --git a/internal/derive/derive.go b/internal/derive/derive.go
index 49265e4..fe9176c 100644
--- a/internal/derive/derive.go
+++ b/internal/derive/derive.go
@@ -2,6 +2,7 @@
 package derive
 
 import (
+	"errors"
 	"fmt"
 
 	"git.eeqj.de/sneak/secret/pkg/bip85"
@@ -16,8 +17,18 @@ const (
 
 	// Size is how many bytes every key type is given.
 	Size = 32
+
+	// MaxIndex is the largest key index there is. Every element of
+	// the path is hardened, and a hardened BIP-32 child index stops
+	// here.
+	MaxIndex = 1<<31 - 1
 )
 
+// ErrIndexTooLarge is returned for a key index above MaxIndex. Such an
+// index has no hardened child to derive, so there is no key to give
+// back rather than a key nobody else would reproduce.
+var ErrIndexTooLarge = errors.New("the key index is too large")
+
 // Path returns the derivation path for an application number and a key
 // index.
 func Path(application, index uint32) string {
@@ -28,7 +39,15 @@ func Path(application, index uint32) string {
 // The mnemonic becomes a seed with an empty passphrase, the seed
 // becomes a master key, the master key gives BIP-85 entropy at the
 // path, and the entropy seeds the generator the bytes are read from.
+// An index above MaxIndex is refused before any of that happens.
 func Bytes(words string, application, index uint32) ([]byte, error) {
+	if index > MaxIndex {
+		return nil, fmt.Errorf(
+			"%w: %d is above %d",
+			ErrIndexTooLarge, index, MaxIndex,
+		)
+	}
+
 	seed := bip39.NewSeed(words, "")
 
 	master, err := hdkeychain.NewMaster(seed, &chaincfg.MainNetParams)
diff --git a/internal/derive/derive_test.go b/internal/derive/derive_test.go
index d5461ac..75f8b70 100644
--- a/internal/derive/derive_test.go
+++ b/internal/derive/derive_test.go
@@ -38,6 +38,16 @@ func TestEveryIndexGivesItsOwnBytes(t *testing.T) {
 	require.False(t, bytes.Equal(first, second))
 }
 
+func TestAnIndexWithNoHardenedChildIsRefused(t *testing.T) {
+	t.Parallel()
+
+	_, err := derive.Bytes(example(), application, derive.MaxIndex+1)
+	require.ErrorIs(t, err, derive.ErrIndexTooLarge)
+
+	_, err = derive.Bytes(example(), application, derive.MaxIndex)
+	require.NoError(t, err)
+}
+
 func TestTheSameInputAlwaysGivesTheSameBytes(t *testing.T) {
 	t.Parallel()
 
diff --git a/script/install-precommit b/script/install-precommit
index bef6406..723bae1 100755
--- a/script/install-precommit
+++ b/script/install-precommit
@@ -8,8 +8,8 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     hook=".git/hooks/pre-commit"
-    printf '#!/bin/sh\nset -e\nscript/precommit\n' > .git/hooks/pre-commit
-    chmod +x .git/hooks/pre-commit
+    printf '#!/bin/sh\nset -e\nscript/precommit\n' > "$hook"
+    chmod +x "$hook"
     echo "pre-commit hook installed: runs script/precommit"
 }