prepare_sql = true (creating prepared queries) makes NHibernate set up wrong size for byte arrays larger than 8000

Mapping:

<class name="Document" table="documents_document" lazy="true" batch-size="10">
<id name="oid" type="System.Guid" unsaved-value="00000000-0000-0000-0000-000000000000" access="field.camelcase">
<generator class="guid.comb" />
</id>
<property name="Contents" access="field.camelcase" type="BinaryBlob" length="20000"></property>
</class>

Field:

private byte[] contents;

When updating the field, NHibernate sets the size into the IDbCommandParameter to 8000, causing the data to be truncated.

Regards
Carsten