zk button using Watir generates error indicating no button is present

zk button using Watir generates error indicating no button is present

I´m testing a zk form, and I want to click a button. My zk code is the
following:
<zk:button id="btnLogin"
label="Entrar" onClick="wndLogin.onLogin()"
style="font-weight: bold; font-size:12px;" />
My watir code is shown below:
browser.button.click
I only have one button on the page. When I run the script, I get an error
that indicates that a button does not exist on the page.