I have 4 text fields that I need to set to null. But am getting an error 'Null is not definied' when trying to use the button. Any feedback as to syntax for this?

Here is the relevant piece of my javascript Onclick button code:

var caseUpdate = new sforce.SObject("Case");

caseUpdate.Id = caseObj.Id;
caseUpdate.InEditUserId__c = Null;
caseUpdate.InEditUser__c =Null;
caseUpdate.InEditTime__c =Null;
caseUpdate.InEditViewURL__c=Null;

updateRecords.push(caseUpdate);

Hi All

I'm trying to add a field of type Formula to a custom object and I'm getting the following error when I hit 'save' at the end of the new field wizard:



Validation Errors While Saving Record(s)

There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Formula has no columnType". 

I can't find doumentation of this error.

Please does anyone know what does this error mean and what can be the cause of it?

Mant thanks