diff --git a/ast.c b/ast.c
index 48db3982ac..cc35a69a22 100644
--- a/ast.c
+++ b/ast.c
@@ -802,7 +802,6 @@ ast_node_script_lines(rb_execution_context_t *ec, VALUE self)
     TypedData_Get_Struct(self, struct ASTNodeData, &rb_node_type, data);
     ast = rb_ruby_ast_data_get(data->vast);
     rb_parser_ary_t *ret = ast->body.script_lines;
-    if (!ret || FIXNUM_P((VALUE)ret)) return Qnil;
     return rb_parser_build_script_lines_from(ret);
 }
 
diff --git a/compile.c b/compile.c
index 077770a7e4..588ce0c7ee 100644
--- a/compile.c
+++ b/compile.c
@@ -1479,7 +1479,7 @@ new_child_iseq(rb_iseq_t *iseq, const NODE *const node,
                VALUE name, const rb_iseq_t *parent, enum rb_iseq_type type, int line_no)
 {
     rb_iseq_t *ret_iseq;
-    VALUE vast = rb_ruby_ast_new(node, NULL);
+    VALUE vast = rb_ruby_ast_new(node);
 
     debugs("[new_child_iseq]> ---------------------------------------\n");
     int isolated_depth = ISEQ_COMPILE_DATA(iseq)->isolated_depth;
@@ -8771,7 +8771,7 @@ compile_builtin_mandatory_only_method(rb_iseq_t *iseq, const NODE *node, const N
     scope_node.nd_body = mandatory_node(iseq, node);
     scope_node.nd_args = &args_node;
 
-    VALUE vast = rb_ruby_ast_new(RNODE(&scope_node), NULL);
+    VALUE vast = rb_ruby_ast_new(RNODE(&scope_node));
 
     ISEQ_BODY(iseq)->mandatory_only_iseq =
       rb_iseq_new_with_opt(vast, rb_iseq_base_label(iseq),
diff --git a/internal/ruby_parser.h b/internal/ruby_parser.h
index eb36be7d4b..98c3461954 100644
--- a/internal/ruby_parser.h
+++ b/internal/ruby_parser.h
@@ -96,7 +96,7 @@ enum lex_state_e {
     EXPR_NONE = 0
 };
 
-VALUE rb_ruby_ast_new(const NODE *const root, rb_parser_ary_t *script_lines);
+VALUE rb_ruby_ast_new(const NODE *const root);
 rb_ast_t *rb_ruby_ast_data_get(VALUE vast);
 
 #endif /* INTERNAL_RUBY_PARSE_H */
diff --git a/iseq.c b/iseq.c
index c07a320620..34381c6a46 100644
--- a/iseq.c
+++ b/iseq.c
@@ -851,14 +851,7 @@ static int
 ast_line_count(const VALUE vast)
 {
     rb_ast_t *ast = rb_ruby_ast_data_get(vast);
-    if (!ast || !ast->body.script_lines) {
-        // this occurs when failed to parse the source code with a syntax error
-        return 0;
-    }
-    if (!FIXNUM_P((VALUE)ast->body.script_lines)) {
-        return (int)ast->body.script_lines->len;
-    }
-    return FIX2INT((VALUE)ast->body.script_lines);
+    return ast->body.line_count;
 }
 
 static VALUE
@@ -999,7 +992,7 @@ rb_iseq_new_with_opt(const VALUE vast, VALUE name, VALUE path, VALUE realpath,
     if (!NIL_P(script_lines)) {
         // noop
     }
-    else if (body && !FIXNUM_P((VALUE)body->script_lines) && body->script_lines) {
+    else if (body && body->script_lines) {
         script_lines = rb_parser_build_script_lines_from(body->script_lines);
     }
     else if (parent) {
diff --git a/node.c b/node.c
index 33a6c0c357..2d3ef6171d 100644
--- a/node.c
+++ b/node.c
@@ -340,6 +340,7 @@ iterate_node_values(rb_ast_t *ast, node_buffer_list_t *nb, node_itr_t * func, vo
 static void
 script_lines_free(rb_ast_t *ast, rb_parser_ary_t *script_lines)
 {
+    if (!script_lines) return;
     for (long i = 0; i < script_lines->len; i++) {
         parser_string_free(ast, (rb_parser_string_t *)script_lines->data[i]);
     }
@@ -358,10 +359,8 @@ rb_ast_free(rb_ast_t *ast)
 #ifdef UNIVERSAL_PARSER
     if (ast && ast->node_buffer) {
         void (*free_func)(void *) = xfree;
-        if (ast->body.script_lines && !FIXNUM_P((VALUE)ast->body.script_lines)) {
-            script_lines_free(ast, ast->body.script_lines);
-            ast->body.script_lines = NULL;
-        }
+        script_lines_free(ast, ast->body.script_lines);
+        ast->body.script_lines = NULL;
         rb_node_buffer_free(ast, ast->node_buffer);
         ast->node_buffer = 0;
         free_func(ast);
@@ -418,7 +417,7 @@ rb_ast_memsize(const rb_ast_t *ast)
         }
     }
 
-    if (script_lines && !FIXNUM_P((VALUE)script_lines)) {
+    if (script_lines) {
         size += sizeof(rb_parser_ary_t);
         for (i = 0; i < script_lines->len; i++) {
             size += sizeof(rb_parser_string_t);
@@ -436,10 +435,8 @@ rb_ast_dispose(rb_ast_t *ast)
     // noop. See the comment in rb_ast_free().
 #else
     if (ast && ast->node_buffer) {
-        if (ast->body.script_lines && !FIXNUM_P((VALUE)ast->body.script_lines)) {
-            script_lines_free(ast, ast->body.script_lines);
-            ast->body.script_lines = NULL;
-        }
+        script_lines_free(ast, ast->body.script_lines);
+        ast->body.script_lines = NULL;
         rb_node_buffer_free(ast, ast->node_buffer);
         ast->node_buffer = 0;
     }
diff --git a/parse.y b/parse.y
index 24f19a683c..4be5512700 100644
--- a/parse.y
+++ b/parse.y
@@ -7755,7 +7755,7 @@ yycompile0(VALUE arg)
         }
     }
     p->ast->body.root = tree;
-    if (!p->ast->body.script_lines) p->ast->body.script_lines = (rb_parser_ary_t *)INT2FIX(p->line_count);
+    p->ast->body.line_count = p->line_count;
     return TRUE;
 }
 
diff --git a/ruby_parser.c b/ruby_parser.c
index b72f875155..63cd8df814 100644
--- a/ruby_parser.c
+++ b/ruby_parser.c
@@ -893,6 +893,7 @@ VALUE
 rb_parser_build_script_lines_from(rb_parser_ary_t *lines)
 {
     int i;
+    if (!lines) return Qnil;
     if (lines->data_type != PARSER_ARY_DATA_SCRIPT_LINE) {
         rb_bug("unexpected rb_parser_ary_data_type (%d) for script lines", lines->data_type);
     }
@@ -1115,7 +1116,7 @@ parser_aset_script_lines_for(VALUE path, rb_parser_ary_t *lines)
 {
     VALUE hash, script_lines;
     ID script_lines_id;
-    if (NIL_P(path) || !lines || FIXNUM_P((VALUE)lines)) return;
+    if (NIL_P(path) || !lines) return;
     CONST_ID(script_lines_id, "SCRIPT_LINES__");
     if (!rb_const_defined_at(rb_cObject, script_lines_id)) return;
     hash = rb_const_get_at(rb_cObject, script_lines_id);
@@ -1126,7 +1127,7 @@ parser_aset_script_lines_for(VALUE path, rb_parser_ary_t *lines)
 }
 
 VALUE
-rb_ruby_ast_new(const NODE *const root, rb_parser_ary_t *script_lines)
+rb_ruby_ast_new(const NODE *const root)
 {
     VALUE vast = ast_alloc();
     rb_ast_t *ast = DATA_PTR(vast);
@@ -1134,7 +1135,8 @@ rb_ruby_ast_new(const NODE *const root, rb_parser_ary_t *script_lines)
         .root = root,
         .frozen_string_literal = -1,
         .coverage_enabled = -1,
-        .script_lines = script_lines
+        .script_lines = NULL,
+        .line_count = 0,
     };
     return vast;
 }
diff --git a/rubyparser.h b/rubyparser.h
index 55a49a2b32..22ac7df088 100644
--- a/rubyparser.h
+++ b/rubyparser.h
@@ -1212,9 +1212,7 @@ typedef struct node_buffer_struct node_buffer_t;
 typedef struct rb_ast_body_struct {
     const NODE *root;
     rb_parser_ary_t *script_lines;
-    // script_lines is either:
-    // - a Fixnum that represents the line count of the original source, or
-    // - an rb_parser_ary_t* that contains the lines of the original source
+    int line_count;
     signed int frozen_string_literal:2; /* -1: not specified, 0: false, 1: true */
     signed int coverage_enabled:2; /* -1: not specified, 0: false, 1: true */
 } rb_ast_body_t;
diff --git a/vm.c b/vm.c
index 4bd95034cc..e2813c9cc0 100644
--- a/vm.c
+++ b/vm.c
@@ -1480,7 +1480,17 @@ rb_binding_add_dynavars(VALUE bindval, rb_binding_t *bind, int dyncount, const I
     tmp_node.nd_body = 0;
     tmp_node.nd_args = 0;
 
-    VALUE vast = rb_ruby_ast_new(RNODE(&tmp_node), (rb_parser_ary_t *)INT2FIX(-1));
+    VALUE vast = rb_ruby_ast_new(RNODE(&tmp_node));
+    {   /*
+         * TODO:
+         * Assigning -1 to line_count is to maintain the previous code.
+         * However, the author of this patch guesses this code is no longer necessary.
+         * We will try to remove this code in the next commit which is atomically
+         * revertable if the author is wrong.
+         */
+        rb_ast_t *ast = (rb_ast_t *)DATA_PTR(vast);
+        ast->body.line_count = -1;
+    }
 
     if (base_iseq) {
         iseq = rb_iseq_new(vast, ISEQ_BODY(base_iseq)->location.label, path, realpath, base_iseq, ISEQ_TYPE_EVAL);