diff --git a/ast.c b/ast.c
index 3fec251bcc..78de316c61 100644
--- a/ast.c
+++ b/ast.c
@@ -710,7 +710,7 @@ ast_node_script_lines(rb_execution_context_t *ec, VALUE self)
     struct ASTNodeData *data;
     TypedData_Get_Struct(self, struct ASTNodeData, &rb_node_type, data);
     VALUE ret = data->ast->body.script_lines;
-    if (!ret) ret = Qnil;
+    if (!RB_TYPE_P(ret, T_ARRAY)) return Qnil;
     return ret;
 }
 
diff --git a/compile.c b/compile.c
index 30c75119b2..4b967a87a0 100644
--- a/compile.c
+++ b/compile.c
@@ -1328,8 +1328,7 @@ new_child_iseq(rb_iseq_t *iseq, const NODE *const node,
 
     ast.root = node;
     ast.compile_option = 0;
-    ast.line_count = -1;
-    ast.script_lines = Qfalse;
+    ast.script_lines = INT2FIX(-1);
 
     debugs("[new_child_iseq]> ---------------------------------------\n");
     int isolated_depth = ISEQ_COMPILE_DATA(iseq)->isolated_depth;
diff --git a/iseq.c b/iseq.c
index 4ad1fc23be..1609432770 100644
--- a/iseq.c
+++ b/iseq.c
@@ -813,13 +813,23 @@ rb_iseq_new(const rb_ast_body_t *ast, VALUE name, VALUE path, VALUE realpath,
                                 0, type, &COMPILE_OPTION_DEFAULT);
 }
 
+static int
+ast_line_count(const rb_ast_body_t *ast)
+{
+    if (RB_TYPE_P(ast->script_lines, T_ARRAY)){
+        return (int)RARRAY_LEN(ast->script_lines);
+    }
+    return FIX2INT(ast->script_lines);
+}
+
 rb_iseq_t *
 rb_iseq_new_top(const rb_ast_body_t *ast, VALUE name, VALUE path, VALUE realpath, const rb_iseq_t *parent)
 {
     VALUE coverages = rb_get_coverages();
     if (RTEST(coverages)) {
-        if (ast->line_count >= 0) {
-            int len = (rb_get_coverage_mode() & COVERAGE_TARGET_ONESHOT_LINES) ? 0 : ast->line_count;
+        int line_count = ast_line_count(ast);
+        if (line_count >= 0) {
+            int len = (rb_get_coverage_mode() & COVERAGE_TARGET_ONESHOT_LINES) ? 0 : line_count;
             VALUE coverage = rb_default_coverage(len);
             rb_hash_aset(coverages, path, coverage);
         }
diff --git a/node.h b/node.h
index 592b285b83..8b09e81a8c 100644
--- a/node.h
+++ b/node.h
@@ -398,8 +398,10 @@ typedef struct node_buffer_struct node_buffer_t;
 typedef struct rb_ast_body_struct {
     const NODE *root;
     VALUE compile_option;
-    int line_count;
     VALUE script_lines;
+    // script_lines is either:
+    // - a Fixnum that represents the line count of the original source, or
+    // - an Array that contains the lines of the original source
 } rb_ast_body_t;
 typedef struct rb_ast_struct {
     VALUE flags;
diff --git a/parse.y b/parse.y
index 47b63e810d..df16cf6236 100644
--- a/parse.y
+++ b/parse.y
@@ -6286,7 +6286,7 @@ yycompile0(VALUE arg)
         RB_OBJ_WRITE(p->ast, &p->ast->body.compile_option, opt);
     }
     p->ast->body.root = tree;
-    p->ast->body.line_count = p->line_count;
+    if (!p->ast->body.script_lines) p->ast->body.script_lines = INT2FIX(p->line_count);
     return TRUE;
 }
 
diff --git a/vm.c b/vm.c
index 01afea9d4d..6140bcf861 100644
--- a/vm.c
+++ b/vm.c
@@ -1223,7 +1223,7 @@ rb_binding_add_dynavars(VALUE bindval, rb_binding_t *bind, int dyncount, const I
     rb_node_init(&tmp_node, NODE_SCOPE, (VALUE)dyns, 0, 0);
     ast.root = &tmp_node;
     ast.compile_option = 0;
-    ast.line_count = -1;
+    ast.script_lines = INT2FIX(-1);
 
     if (base_iseq) {
 	iseq = rb_iseq_new(&ast, base_iseq->body->location.label, path, realpath, base_iseq, ISEQ_TYPE_EVAL);