How to use PostgreSql hstore type with Fluent NHibarnate in C #

Question

I have an mvc application that uses free nhibarnate that connects to the postgresql database. I have problems when I try to get the hstore type from the database. And I'm trying to change the essence of how

An object

IDictionary<string,string> fields {get;set;} 

Mapping

  Map(x=>x.fields).CustomSqlType("hstore"); 

Getting an error, for example, Failed to determine the type for: System.Collections.Generic.IDictionary`2

So what is the way to get a field like hstore from the postgresql database?