diff --git a/internal/language/language.go b/internal/language/language.go
index 09d41c73..548358be 100644
--- a/internal/language/language.go
+++ b/internal/language/language.go
@@ -164,7 +164,12 @@ func (t Tag) MarshalText() (text []byte, err error) {
 
 // UnmarshalText implements encoding.TextUnmarshaler.
 func (t *Tag) UnmarshalText(text []byte) error {
-	tag, err := Parse(string(text))
+	s := string(text)
+	if strings.Count(s, "-")+strings.Count(s, "_") > 1000 {
+		*t = Und
+		return ErrSyntax
+	}
+	tag, err := Parse(s)
 	*t = tag
 	return err
 }
diff --git a/internal/language/language_test.go b/internal/language/language_test.go
index 269179c2..56c0f18d 100644
--- a/internal/language/language_test.go
+++ b/internal/language/language_test.go
@@ -6,6 +6,7 @@ package language
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 
 	"golang.org/x/text/internal/testtext"
@@ -105,6 +106,19 @@ func TestMarshal(t *testing.T) {
 	}
 }
 
+func TestUnmarshalTextTooBig(t *testing.T) {
+	for _, separator := range []string{"-", "_"} {
+		text := []byte("en" + strings.Repeat(separator+"abcdefghi", 1001))
+		tag := Tag{LangID: _fr}
+		if err := tag.UnmarshalText(text); err != ErrSyntax {
+			t.Errorf("UnmarshalText with %q separators: got error %v; want %v", separator, err, ErrSyntax)
+		}
+		if got := tag.String(); got != "und" {
+			t.Errorf("UnmarshalText with %q separators: got tag %q; want und", separator, got)
+		}
+	}
+}
+
 func TestParseBase(t *testing.T) {
 	tests := []struct {
 		in  string