Disabling a field based on a value in same table

I think i have a very simple question.
I have a table called 'd_pics'. In this table are 3 fields:
PicID [int] [PRIMARY KEY]

Pictype [varchar][LOOK UP from LIST OF VALUES: value 1= Card, value 2= Online]

SerialNr [varchar] {see question]
Now if in the form the value 'Online' is selected, the field 'SerialNr' should be disabled.

But if in the form the value 'Card' is selected, the user should fill out a value at the field 'SerialNr'.
I've tried the code below as I found in http://www.asprunner...drop-down-box/. But it doesn't work and is also based on the fact that the second field (in my case SerialNr) would be a dropdown. In my case the user should fill out something by him selves.
var tName = 'd_pics';

var ctrlPicType = Runner.controls.ControlManager.getAt(tName, pageid, 'PicType');

var ctrlSerialNr = Runner.controls.ControlManager.getAt(tName, pageid, 'SerialNr');
ctrlCountry.on('change', function(e){

if (this.getValue() == 'Card'){

ctrlState.setEnabled();

ctrlState.addValidation("IsRequired");

}else{

ctrlState.setDisabled();

ctrlState.setValue("");

ctrlState.removeValidation("IsRequired");

}

});