Use TokenTypeInvalid as zero value Token kind

encoder.go

@@ -119,6 +119,8 @@ func (thiz *Encoder) Reset(w io.Writer) {
 // of this Encoder.
 func (thiz *Encoder) EncodeToken(t *Token) error {
 	switch t.Kind {
+	case TokenTypeInvalid:
+		return errors.New("trying to encode invalid/zerovalue token")
 	case TokenTypeStartElement:
 		err := thiz.encodeStartElement(t)
 		if err != nil {

namespaceModifier.go

@@ -205,6 +205,9 @@ func (thiz *NamespaceModifier) addPrefixRewrite(original, prefix []byte) {
 // will remain unmodified for the lexical scope of the XML element represented
 // by that token, as per the documentation of EncoderMiddleware.EncodeToken.
 func (thiz NamespaceModifier) NamespaceOfToken(t *Token) []byte {
+	if t.Kind == TokenTypeInvalid {
+		return nil
+	}
 	prefix := t.Name.Prefix
 	if len(prefix) > 0 {
 		alias := thiz.findPrefixAlias(t.Name.Prefix)

types.go

@@ -17,7 +17,8 @@ type Attr struct {
 
 // constants for Token.Kind
 const (
-	TokenTypeStartElement = iota
+	TokenTypeInvalid = iota
+	TokenTypeStartElement
 	TokenTypeEndElement
 	TokenTypeProcInst
 	TokenTypeDirective