The length of the string value exceeds the length configured in the mapping/parameter

I am trying to insert some very long text into a string prop - it worked perfectly fine with LinqToSql, now I have switched over to NHibernate and want to save the same entity, but nHibernate throws the above exception.

How can I fix this?

Originally my props were defined as:

        Map(x => x.Content, "fT_Content").Nullable();
        Map(x => x.Fields, "fT_Fields").Nullable();

now they are: this works but why do I have to do this?

        Map(x => x.Content, "fT_Content").CustomSqlType("nvarchar(max)").Length(Int32.MaxValue).Nullable();
        Map(x => x.Fields, "fT_Fields").CustomSqlType("nvarchar(max)").Length(Int32.MaxValue).Nullable();

Note: I have the latest nhibernate using nuget.

For ref here are the fields:

    public virtual string Content
    {
        get;
        set;
    }

    public virtual string Fields
    {
        get;
        set;
    }

I want to avoid going to live production and all of a sudden inserts stop working on this table....

This is a well known issue with NHibernate handling nvarchar(max), see :

https://web.archive.org/web/20201023061636/http://geekswithblogs.net/lszk/archive/2011/07/11/nhibernatemapping-a-string-field-as-nvarcharmax-in-sql-server-using.aspx

For some years now, I have been file mapping nvarchar(max) columns to StringClob without encountering any problem :

<property name="myProp" column="MY_PROP" not-null="true" type="StringClob" access="property"></property>   

This link (from the comments) describes the fluent mapping required to fix this issue:

https://web.archive.org/web/20201029161314/https://www.tritac.com/nl/blog/fluent-nhibernate-nvarchar-max-fields-truncated-to-4000-characters/

Map(x => x.Description).CustomType("StringClob").CustomSqlType("nvarchar(max)");