use angular ngDisable with kendo mobile

I'm try use ng-disable in angular + kendo mobile but it is not work for me.

I try this code: 

<kendo-mobile-button id="buy-now" ng-disabled="isPlaceOrderEnable()" ng-click="menu.addToCart(kendoEvent)">Place Order</kendo-mobile-button>

the 'isPlaceOrderEnable' function defined in the controller and return true/false.

but the button is always enebled.

Thanks in advance.

Naomi.