i want to make a button disabled if a datagrid is empty and it should be enabled when there is atleast 1 entry.the entries in the grid are made at runtime.I tried this this is the button:

<mx:Button id="update" label="Update Contact" enabled="{isButtonEnabled()}"/>

and the function is defined as where dg_contact is the datagrid:

public function isButtonEnabled():Boolean
{
     if(dg_contact.selectedIndex==-1)
    {
        return false;
    }
    else
    {
        return true;
    }
}

where am i going wrong?