C#/NHibernate: Association references unmapped class

I have been struggling with this NHibernate issue for hours. I extensively researched on the web as well as the NHibernate documentation, and I can make no sense of this issue. I'm relatively new to NHibernate, and love it. In that case, though, it's driving me mad.

I'm writing a small "Poll" module for a website. I have several classes (Poll, PollVote and PollAnswer). The main one, Poll, is the one causing the issue. This this what the class looks like:

public class Poll
    {
        public virtual int Id { get; set; }
        public virtual Site Site { get; set; }
        public virtual string Question { get; set; }
        public virtual bool Locked { get; set; }
        public virtual bool Enabled { get; set; }
        public virtual ICollection<PollAnswer> AnswersSet { get; set; }
        public virtual ICollection<PollVote> VotesSet { get; set; }
    }

And this is what the mapping looks like:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="Folke"
                   namespace="Folke.Code.Domain">
  <class name="Poll">
    <id name="Id">
      <generator class="native"></generator>
    </id>
    <property name="Site"/>
    <property name="Question"/>
    <property name="Locked"/>
    <property name="Enabled"/>
    <set name="AnswersSet" lazy="true">
      <key column="PollId"/>
      <one-to-many class="PollAnswer"/>
    </set>
    <set name="VotesSet" lazy="true">
      <key column="PollId"/>
      <one-to-many class="PollVote"/>
    </set>
  </class>
</hibernate-mapping>

This returns me an error:

Association references unmapped class: Folke.Code.Domain.PollAnswer Description: An unhandled exception occurred during the execution of the current