diff --git a/defaults.go b/defaults.go
index 6d7e17605..a67ee3cbc 100644
--- a/defaults.go
+++ b/defaults.go
@@ -5,7 +5,6 @@ import (
 	"net"
 	"strconv"
 	"strings"
-	"unicode"
 )
 
 const hexDigit = "0123456789abcdef"
@@ -330,19 +329,15 @@ func Fqdn(s string) string {
 }
 
 // CanonicalName returns the domain name in canonical form. A name in canonical
-// form is lowercase and fully qualified. See Section 6.2 in RFC 4034.
-// According to the RFC all uppercase US-ASCII letters in the owner name of the
-// RR areeplaced by the corresponding lowercase US-ASCII letters.
+// form is lowercase and fully qualified. Only US-ASCII letters are affected. See
+// Section 6.2 in RFC 4034.
 func CanonicalName(s string) string {
-	var result strings.Builder
-	for _, ch := range s {
-		if unicode.IsUpper(ch) && (ch >= 0x00 && ch <= 0x7F) {
-			result.WriteRune(unicode.ToLower(ch))
-		} else {
-			result.WriteRune(ch)
+	return strings.Map(func(r rune) rune {
+		if r >= 'A' && r <= 'Z' {
+			r += 'a' - 'A'
 		}
-	}
-	return Fqdn(result.String())
+		return r
+	}, Fqdn(s))
 }
 
 // Copied from the official Go code.