How to save data in ItemFieldValue of custom lookup field type?

Question

I have created custom field type inherited from SPFieldText successfully. I save a data in the item value as string of the next format: "id;#title". Now I need to convert the base type from SPFieldText to SPFieldLookup. But I have a problem with saving values in ItemFieldValue when updating field.

Here is my code of updating SPListItem:

   public override void UpdateFieldValueInItem()
   {
        base.UpdateFieldValueInItem();
        try
        {

            EnsureChildControls();

            var field = Field as RecordLookupField; // my custom field 

            if (!String.IsNullOrEmpty(_lookupFieldValues.Value) && 
                !String.IsNullOrEmpty(_mainTextBx.Text))
            {                  
                if (field != null)
                    Item[field.RelatedField] = SPContext.Current.Web.Lists[Settings.ListId].
                                 Fields[Settings.MainDisplayColumn].InternalName;

                var id = Convert.ToInt32(_lookupFieldId.Value);
                var value =  _lookupFieldValue.Value;
                ItemFieldValue = new SPFieldLookupValue(id,value).ToString(); 
            }
            else
            {
                ItemFieldValue = null;
                Item[Field.RelatedField] = null;
            }
        }
        catch (Exception ex)
        {
            ex.Trace(GetType());
        }
    }

This code works without exception. But when i try to save my field in edit mode I get the following exception: <nativehr>0x80020005</nativehr><nativestack></nativestack>Invalid data has been used to update the list item. The field you are trying to update may be read only.

But when my custom field type inherites from SPFieldText I don't get any exception. How to save lookup value in ItemFieldValue correctly when I inherites from SPFieldLookup ?