getdirty is Undefined.

I am trying to call the function below from OnSave but I keep getting: 

There was an error with this field's customized event.

Field:crmForm

Event:onsave

Error:Unable to get value of the property 'getIsDirty': object is null or undefined.

and yes, I did check the box on the event to pass the context.

function form_OnSave(context)
{
    var zipDirty = Xrm.Page.getAttribute("address_postalcode").getIsDirty();
    var saveMode = context.getEventArgs().getSaveMode();
    alert(saveMode);
    if (zipDirty == true)
    {
        context.getEventArgs().preventDefault();
        //Xrm.Page.ui.controls.get("subject").setFocus();
        setTimeout(function () { Xrm.Page.data.entity.save(saveMode); }, 1000);
        //alert("Unable to save try saving again.");
          }
}  

Why is this and do you know any way of delaying the OnSave so my REST call completes?