diff --git a/lexer/error.go b/lexer/error.go
index 6ffd479..bcc2de4 100644
--- a/lexer/error.go
+++ b/lexer/error.go
@@ -1,6 +1,7 @@
 package lexer
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 )
@@ -9,6 +10,11 @@ var (
 	_ error = (*Error)(nil)
 )
 
+var (
+	// ErrUnacceptableRune indicates the read rune
+	ErrUnacceptableRune = errors.New("rune not acceptable in context")
+)
+
 // Error represents a generic parsing error
 type Error struct {
 	Filename string