a caption inside a borderlayout causes a javascript error

Steps to Reproduce

1. load the zul

<zk>
    <borderlayout>
        <west  >
        </west>
        <center>
        </center>
        <east >
            <caption> EAST 1
            </caption>
        </east>
    </borderlayout>
</zk>

Current Result

Uncaught TypeError: p.getClosableIconClass_ is not a function
at init.caption$mold$ (VM3495 zk.wpd:33956)
at init.redraw (VM3495 zk.wpd:20252)
at init.titleRenderer_ (VM3495 zk.wpd:37651)
at init.layoutregion$mold$ (VM3530 zul.layout.wpd:1599)
at init.redraw (VM3495 zk.wpd:20252)
at init.borderlayout$mold$ (VM3530 zul.layout.wpd:358)
at init.redraw (VM3495 zk.wpd:20252)
at init.layoutregion$mold$ (VM3530 zul.layout.wpd:1607)
at init.redraw (VM3495 zk.wpd:20252)

Expected Result

no error

Debug Information

1. this zul works without errors in 8.5.1.2