How to show/hide a field based on another field.

I have the following document level script:

function Openn(){
        if(this.getField("cobuyer").value.length == 0)
        {
           this.getField("xcobuy").display = display.hidden;
        }

     else
        {
            this.getField("xcobuy").display = display.visible;
        }

                }
Openn();

I am getting the following error:

Acrobat EScript Built-in Functions Version 10.0
Acrobat SOAP 10.0

TypeError: this.getField("xcobuy") is null
9:Document-Level:Openn

The form is being produced with the data already in it and that field is not null

Thanks

Jasper