不允许将元素搜索作为元素导航的子项

问：

W3 validator 说：

Element search not allowed as child of element nav in this context. (Suppressing further errors from this subtree.)

From line 38, column 12457; to line 38, column 12464

Contexts in which element search may be used:
    Where flow content is expected.
Content model for element nav:
    Flow content.

目前的结构是（简化的）：

html
--body
----header
------nav
--------ul
--------search
----------form
------------label
------------input
------------button

我已经尝试将我的搜索表单放在 nav、div 甚至菜单的最后一行中。我每次都收到验证错误。这是怎么回事？我应该把搜索元素放在哪里？