precompiled web with nested user controls

[CBW]

I have a user control called "field.ascx" that is dynamically created
by another user control "form.ascx". I dynamically create it and
reference it in the codebehind of form.ascx using

(ASP.lbt_field)LoadControl("field.ascx")

This works fine when I build the website and run it until I try to
publish the website. Then I get an error

Error 1 The type or namespace name 'lbt_field' does not exist in the
namespace 'ASP' (are you missing an assembly reference?)

My user control field is defined with:
<%@ Control Language="C#" AutoEventWireup="true"
CodeFile="field.ascx.cs" classname="lbt_field"
Inherits="webUI_UserControls_field" %>