CustomTypeHandler seems to doesn't work correctly

I have an entity like this:

[Table("Entity", Schema = "dbo")]
internal class Entity
{
    [Key]
    public virtual int Id { get; set; }
    public virtual CustomType Filters { get; set; }
    public virtual string Name { get; set; }
    ...
}

and custom type like this:

[JsonObject]
public class CustomType
{
    [JsonProperty(PropertyName = "a")]public string a{ get; set; }
    [JsonProperty(PropertyName = "b")]public string b { get; set; }
    [JsonProperty(PropertyName = "d")]public IReadOnlyCollection<d> d{ get; set; }
}

custom handler like this:

public class JsonTypeHandler<CustomType> : SqlMapper.TypeHandler<CustomType>
{
    public override void SetValue(IDbDataParameter parameter, CustomType value)
    {
        parameter.Value = value == null
            ? (object) DBNull.Value
            : JsonConvert.SerializeObject(value);
        parameter.DbType = DbType.String;
    }

    public override CustomType Parse(object value)
    {
        return JsonConvert.DeserializeObject<CustomType>(value.ToString());
    }
}

and I register it in global.asax.cs like this:

SqlMapper.AddTypeHandler(new JsonTypeHandler<CustomType>());

The problem is that SetValue is not invoked when Execute is called. I'm using a Dapper.FastCrud, and my code to insert an entity to db looks like this:

var entity = new Entity
{
    Filters = new CustomType{bla bla},
    Name = "fafsd",
    ...
};

_dbConnection.Insert(dataExtractEntry, x => x.AttachToTransaction(tx));

Is there something more I have to do here?

Start with the JsonTypeHandler registration in global.asax.cs and the Dapper.FastCrud Insert call shown in the issue. Reproduce the insert and trace whether SetValue is reached for Entity.Filters; done means the custom handler is invoked and the CustomType value is persisted correctly.