diff --git a/ast/lex.go b/ast/lex.go
index 3c3bf4c..532f32a 100644
--- a/ast/lex.go
+++ b/ast/lex.go
@@ -26,6 +26,10 @@ const string_delims string = "\"'`"
 
 func Lex(input string) *Token {
     ret := lex(input)
+    if ret == nil {
+        return nil
+    }
+
     if ret.Tag != LIST {
         temp := &Token{Tag: LIST}
         temp.Direct(ret)
@@ -67,6 +71,9 @@ func lex(input string) *Token {
             } else if StrIsNumber(tok) {
                 (*iter).Tag = NUMBER
 
+            } else if tok == "T" || tok == "F" {
+                (*iter).Tag = BOOL
+
             } else {
                 (*iter).Tag = SYMBOL
             }