Mo. Rev. Stat. § 429.005: Certain agreements to waive rights under this chapter are unenforceable — exception.

1. An agreement by an original contractor, subcontractor, supplier or laborer to waive any right to enforce or claim any lien authorized under this chapter, where the agreement is in anticipation of and in consideration for the awarding of a contract or subcontract to perform work or supply materials for an improvement upon real property, whether expressly stated or implied, is against public policy and shall be unenforceable. The provisions of this section shall not prohibit subordination or release of a lien authorized under this chapter.

2. Nothing contained in this section shall be construed to prohibit contractual provisions requiring lien waivers as a condition for payment.