diff --git a/node.c b/node.c
index d64b6d282f..7cd6aee9f2 100644
--- a/node.c
+++ b/node.c
@@ -1235,7 +1235,7 @@ struct node_buffer_struct {
 node_buffer_t *
 rb_node_buffer_new()
 {
-    node_buffer_t *nb = xmalloc(sizeof(node_buffer_t) + 16 * sizeof(NODE));
+    node_buffer_t *nb = xmalloc(offsetof(node_buffer_t, body) + offsetof(node_buffer_elem_t, buf) + 16 * sizeof(NODE));
     nb->idx = 0;
     nb->len = 16;
     nb->head = &nb->body;
@@ -1263,7 +1263,7 @@ rb_ast_newnode(rb_ast_t *ast)
     if (nb->idx >= nb->len) {
 	long n = nb->len * 2;
 	node_buffer_elem_t *nbe;
-	nbe = xmalloc(sizeof(node_buffer_elem_t) + n * sizeof(NODE));
+	nbe = xmalloc(offsetof(node_buffer_elem_t, buf) + n * sizeof(NODE));
 	nb->idx = 0;
 	nb->len = n;
 	nbe->next = nb->head;