diff --git a/symbol.go b/symbol.go
index b05e560..9f9f152 100644
--- a/symbol.go
+++ b/symbol.go
@@ -90,6 +90,7 @@ const (
 	SymKwBut
 	SymKwFunc
 	SymKwBuiltin
+	SymKwNil
 )
 
 var keywords map[string]Symbol
@@ -103,5 +104,6 @@ func init() {
 		"FUNC":    SymKwFunc,
 		"NOT":     SymKwNot,
 		"OR":      SymKwOr,
+		"NIL":     SymKwNil,
 	}
 }