diff --git a/lexer.go b/lexer.go
index 63ae242..a3057c4 100644
--- a/lexer.go
+++ b/lexer.go
@@ -44,6 +44,7 @@ const (
 func (t TokenType) String() string {
 	types := []string{
 		"EOF",
+		"Undefined",
 		"TagStart",
 		"TagEnd",
 		"TagKey",
diff --git a/pgn.go b/pgn.go
index e9fa8ea..5947e9d 100644
--- a/pgn.go
+++ b/pgn.go
@@ -519,7 +519,7 @@ func (p *Parser) parseVariation() error {
 		// the last move before the variation start
 		variationParent = parentMove.parent
 		// Reset position to where the variation starts
-		if variationParent.parent != nil {
+		if variationParent.parent != nil && variationParent.parent.position != nil {
 			p.game.pos = variationParent.parent.position.copy()
 			if newPos := p.game.pos.Update(variationParent); newPos != nil {
 				p.game.pos = newPos
diff --git a/polyglot.go b/polyglot.go
index af56927..ae6f2ed 100644
--- a/polyglot.go
+++ b/polyglot.go
@@ -194,12 +194,12 @@ func LoadFromSource(source BookSource) (*PolyglotBook, error) {
 
 	buf := make([]byte, 16)
 	for {
-		_, err := source.Read(buf)
-		if err == io.EOF {
+		_, readErr := source.Read(buf)
+		if readErr == io.EOF {
 			break
 		}
-		if err != nil {
-			return nil, err
+		if readErr != nil {
+			return nil, readErr
 		}
 
 		entry := PolyglotEntry{
diff --git a/polyglot_test.go b/polyglot_test.go
index e5be5e4..01a867c 100644
--- a/polyglot_test.go
+++ b/polyglot_test.go
@@ -3,6 +3,7 @@ package chess
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"io"
 	"os"
 	"path/filepath"
@@ -60,7 +61,7 @@ func TestBytesBookSource(t *testing.T) {
 	// Test EOF
 	source.index = 32
 	n, err = source.Read(buf)
-	if err != io.EOF {
+	if !errors.Is(err, io.EOF) {
 		t.Errorf("Read() error = %v, want EOF", err)
 	}
 }