diff --git a/ext/zorder/zorder.go b/ext/zorder/zorder.go
index 627b632..f95c605 100644
--- a/ext/zorder/zorder.go
+++ b/ext/zorder/zorder.go
@@ -19,9 +19,9 @@ func Register(db *sqlite3.Conn) error {
 }
 
 func zorder(ctx sqlite3.Context, arg ...sqlite3.Value) {
-	var x [63]int64
-	if len(arg) > len(x) {
-		ctx.ResultError(util.ErrorString("zorder: too many parameters"))
+	var x [24]int64
+	if n := len(arg); n < 2 || n > 24 {
+		ctx.ResultError(util.ErrorString("zorder: needs between 2 and 24 dimensions"))
 		return
 	}
 	for i := range arg {
@@ -29,17 +29,15 @@ func zorder(ctx sqlite3.Context, arg ...sqlite3.Value) {
 	}
 
 	var z int64
-	if len(arg) > 0 {
-		for i := range x {
-			j := i % len(arg)
-			z |= (x[j] & 1) << i
-			x[j] >>= 1
-		}
+	for i := range 63 {
+		j := i % len(arg)
+		z |= (x[j] & 1) << i
+		x[j] >>= 1
 	}
 
 	for i := range arg {
 		if x[i] != 0 {
-			ctx.ResultError(util.ErrorString("zorder: parameter too large"))
+			ctx.ResultError(util.ErrorString("zorder: argument out of range"))
 			return
 		}
 	}
@@ -51,6 +49,19 @@ func unzorder(ctx sqlite3.Context, arg ...sqlite3.Value) {
 	n := arg[1].Int64()
 	z := arg[0].Int64()
 
+	if n < 2 || n > 24 {
+		ctx.ResultError(util.ErrorString("unzorder: needs between 2 and 24 dimensions"))
+		return
+	}
+	if i < 0 || i >= n {
+		ctx.ResultError(util.ErrorString("unzorder: index out of range"))
+		return
+	}
+	if z < 0 {
+		ctx.ResultError(util.ErrorString("unzorder: argument out of range"))
+		return
+	}
+
 	var k int
 	var x int64
 	for j := i; j < 63; j += n {
diff --git a/ext/zorder/zorder_test.go b/ext/zorder/zorder_test.go
index 4dbad49..01537b1 100644
--- a/ext/zorder/zorder_test.go
+++ b/ext/zorder/zorder_test.go
@@ -12,7 +12,7 @@ import (
 	"github.com/ncruces/go-sqlite3/vfs/memdb"
 )
 
-func TestRegister_zorder(t *testing.T) {
+func Test_zorder(t *testing.T) {
 	t.Parallel()
 	tmp := memdb.TestDB(t)
 
@@ -57,7 +57,7 @@ func TestRegister_zorder(t *testing.T) {
 	}
 }
 
-func TestRegister_unzorder(t *testing.T) {
+func Test_unzorder(t *testing.T) {
 	t.Parallel()
 	tmp := memdb.TestDB(t)
 
@@ -85,7 +85,7 @@ func TestRegister_unzorder(t *testing.T) {
 	}
 }
 
-func TestRegister_error(t *testing.T) {
+func Test_zorder_error(t *testing.T) {
 	t.Parallel()
 	tmp := memdb.TestDB(t)
 
@@ -103,7 +103,7 @@ func TestRegister_error(t *testing.T) {
 
 	var buf strings.Builder
 	buf.WriteString("SELECT zorder(0")
-	for i := 1; i < 80; i++ {
+	for i := 1; i < 25; i++ {
 		buf.WriteByte(',')
 		buf.WriteString(strconv.Itoa(0))
 	}
@@ -113,3 +113,30 @@ func TestRegister_error(t *testing.T) {
 		t.Error("want error")
 	}
 }
+
+func Test_unzorder_error(t *testing.T) {
+	t.Parallel()
+	tmp := memdb.TestDB(t)
+
+	db, err := driver.Open(tmp, zorder.Register)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	var got int64
+	err = db.QueryRow(`SELECT unzorder(-1, 2, 0)`).Scan(&got)
+	if err == nil {
+		t.Error("want error")
+	}
+
+	err = db.QueryRow(`SELECT unzorder(0, 2, 2)`).Scan(&got)
+	if err == nil {
+		t.Error("want error")
+	}
+
+	err = db.QueryRow(`SELECT unzorder(0, 25, 2)`).Scan(&got)
+	if err == nil {
+		t.Error("want error")
+	}
+}