Could not find the property - exception after switching from NHibernate 3 to 3.3.1 - nhibernate

Question

I have a class with a field:
protected DateTime insertDate;
This is mapping for this fiels:
<property name="InsertDate" access="field.camelcase" update="false" />
This field is set when with ctor
public DocuBase(DateTime insertDate)
and is persisted only when row is added to the database. I don't need property for it at all, no setter & no getter. I worked in NHibernate 3.
And now, I've moved from NHiberbate 3 to NHibernate 3.3.1, and I get this exception when session factory is created:
Could not find the property 'InsertDate', associated to the field
'insertDate', in class 'XXXX'
Why is is happening & how can I change mapping in order to get rid of the exception?
EDIT: Below answer is completly correct. But for those of you that don't need/don't want to have a property, and only field, there's another solution:
set name attribute to field name (in my case it is insertDate) and remember to have correct column name
<property name="insertDate" column="InsertDate" access="field.camelcase" update="false" />

Answer 1

It is case sensitivity, this will work.
<property name="insertDate" column="InsertDate" update="false" />
Looks like in release 3.1.0, there was a breaking change
NH today accepts code below. It would be better if this would throw - it causes problem when configurate NH (or 3rd party tools) other ways than by hbm, using the property name (or memberinfo) of the public interface.
[hbm]
<property name="Name" access="field.camelcase" />
[code]
string name;
public virtual string SomeThingCompletelyDifferent{
get {return name;}
set{name=value;}
}
Note: This will be a breaking change.
ps - updated answer to remove reference to use Property with private set as this was not what was being looked for and above breaking change is more relevant.