diff --git a/src/strings/builder.go b/src/strings/builder.go
index 3caddabd4e..096e9c765e 100644
--- a/src/strings/builder.go
+++ b/src/strings/builder.go
@@ -103,18 +103,9 @@ func (b *Builder) WriteByte(c byte) error {
 // It returns the length of r and a nil error.
 func (b *Builder) WriteRune(r rune) (int, error) {
 	b.copyCheck()
-	// Compare as uint32 to correctly handle negative runes.
-	if uint32(r) < utf8.RuneSelf {
-		b.buf = append(b.buf, byte(r))
-		return 1, nil
-	}
-	l := len(b.buf)
-	if cap(b.buf)-l < utf8.UTFMax {
-		b.grow(utf8.UTFMax)
-	}
-	n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r)
-	b.buf = b.buf[:l+n]
-	return n, nil
+	n := len(b.buf)
+	b.buf = utf8.AppendRune(b.buf, r)
+	return len(b.buf) - n, nil
 }
 
 // WriteString appends the contents of s to b's buffer.