diff --git a/ext/rbs_extension/parser.c b/ext/rbs_extension/parser.c
index cafb4a0d5..c6f89a1fc 100644
--- a/ext/rbs_extension/parser.c
+++ b/ext/rbs_extension/parser.c
@@ -2836,7 +2836,10 @@ parse_type_try(VALUE a) {
 static VALUE
 rbsparser_parse_type(VALUE self, VALUE buffer, VALUE start_pos, VALUE end_pos, VALUE variables, VALUE require_eof)
 {
-  parserstate *parser = alloc_parser(buffer, FIX2INT(start_pos), FIX2INT(end_pos), variables);
+  VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
+  StringValue(string);
+  lexstate *lexer = alloc_lexer(string, FIX2INT(start_pos), FIX2INT(end_pos));
+  parserstate *parser = alloc_parser(buffer, lexer, FIX2INT(start_pos), FIX2INT(end_pos), variables);
   struct parse_type_arg arg = {
     parser,
     require_eof
@@ -2864,7 +2867,10 @@ parse_method_type_try(VALUE a) {
 static VALUE
 rbsparser_parse_method_type(VALUE self, VALUE buffer, VALUE start_pos, VALUE end_pos, VALUE variables, VALUE require_eof)
 {
-  parserstate *parser = alloc_parser(buffer, FIX2INT(start_pos), FIX2INT(end_pos), variables);
+  VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
+  StringValue(string);
+  lexstate *lexer = alloc_lexer(string, FIX2INT(start_pos), FIX2INT(end_pos));
+  parserstate *parser = alloc_parser(buffer, lexer, FIX2INT(start_pos), FIX2INT(end_pos), variables);
   struct parse_type_arg arg = {
     parser,
     require_eof
@@ -2881,13 +2887,18 @@ parse_signature_try(VALUE a) {
 static VALUE
 rbsparser_parse_signature(VALUE self, VALUE buffer, VALUE end_pos)
 {
-  parserstate *parser = alloc_parser(buffer, 0, FIX2INT(end_pos), Qnil);
+  VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
+  StringValue(string);
+  lexstate *lexer = alloc_lexer(string, 0, FIX2INT(end_pos));
+  parserstate *parser = alloc_parser(buffer, lexer, 0, FIX2INT(end_pos), Qnil);
   return rb_ensure(parse_signature_try, (VALUE)parser, ensure_free_parser, (VALUE)parser);
 }
 
 static VALUE
 rbsparser_lex(VALUE self, VALUE buffer, VALUE end_pos) {
-  lexstate *lexer = alloc_lexer(buffer, 0, FIX2INT(end_pos));
+  VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
+  StringValue(string);
+  lexstate *lexer = alloc_lexer(string, 0, FIX2INT(end_pos));
   VALUE results = rb_ary_new();
 
   token token = NullToken;
@@ -2906,6 +2917,7 @@ rbsparser_lex(VALUE self, VALUE buffer, VALUE end_pos) {
 
 void rbs__init_parser(void) {
   RBS_Parser = rb_define_class_under(RBS, "Parser", rb_cObject);
+  rb_gc_register_mark_object(RBS_Parser);
   rb_define_singleton_method(RBS_Parser, "_parse_type", rbsparser_parse_type, 5);
   rb_define_singleton_method(RBS_Parser, "_parse_method_type", rbsparser_parse_method_type, 5);
   rb_define_singleton_method(RBS_Parser, "_parse_signature", rbsparser_parse_signature, 2);
diff --git a/ext/rbs_extension/parserstate.c b/ext/rbs_extension/parserstate.c
index 58e4b0206..4818f05a7 100644
--- a/ext/rbs_extension/parserstate.c
+++ b/ext/rbs_extension/parserstate.c
@@ -274,11 +274,7 @@ VALUE comment_to_ruby(comment *com, VALUE buffer) {
   );
 }
 
-lexstate *alloc_lexer(VALUE buffer, int start_pos, int end_pos) {
-  VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
-
-  StringValue(string);
-
+lexstate *alloc_lexer(VALUE string, int start_pos, int end_pos) {
   if (start_pos < 0 || end_pos < 0) {
     rb_raise(rb_eArgError, "negative position range: %d...%d", start_pos, end_pos);
   }
@@ -295,8 +291,7 @@ lexstate *alloc_lexer(VALUE buffer, int start_pos, int end_pos) {
   return lexer;
 }
 
-parserstate *alloc_parser(VALUE buffer, int start_pos, int end_pos, VALUE variables) {
-  lexstate *lexer = alloc_lexer(buffer, start_pos, end_pos);
+parserstate *alloc_parser(VALUE buffer, lexstate *lexer, int start_pos, int end_pos, VALUE variables) {
   parserstate *parser = calloc(1, sizeof(parserstate));
   parser->lexstate = lexer;
   parser->buffer = buffer;
diff --git a/ext/rbs_extension/parserstate.h b/ext/rbs_extension/parserstate.h
index 4f495dbbe..4e443f19c 100644
--- a/ext/rbs_extension/parserstate.h
+++ b/ext/rbs_extension/parserstate.h
@@ -97,20 +97,21 @@ bool parser_typevar_member(parserstate *state, ID id);
  * Allocate new lexstate object.
  *
  * ```
- * alloc_lexer(buffer, 0, 31)    // New lexstate with buffer
+ * VALUE string = rb_funcall(buffer, rb_intern("content"), 0);
+ * alloc_lexer(string, 0, 31)    // New lexstate with buffer content
  * ```
  * */
-lexstate *alloc_lexer(VALUE buffer, int start_pos, int end_pos);
+lexstate *alloc_lexer(VALUE string, int start_pos, int end_pos);
 
 /**
  * Allocate new parserstate object.
  *
  * ```
- * alloc_parser(buffer, 0, 1, variables)    // New parserstate with variables
- * alloc_parser(buffer, 3, 5, Qnil)         // New parserstate without variables
+ * alloc_parser(buffer, lexer, 0, 1, variables)    // New parserstate with variables
+ * alloc_parser(buffer, lexer, 3, 5, Qnil)         // New parserstate without variables
  * ```
  * */
-parserstate *alloc_parser(VALUE buffer, int start_pos, int end_pos, VALUE variables);
+parserstate *alloc_parser(VALUE buffer, lexstate *lexer, int start_pos, int end_pos, VALUE variables);
 void free_parser(parserstate *parser);
 /**
  * Advance one token.