Nested Master Pages: Accessing Properties

[Diane Yocom]

I have two nested master pages, each with public properties. From my
content page, I am able to access properties from both pages using either
Master.PropertyName (for the child master page) or
Master.Master.PropertyName (for the parent master page). The solution
compiles and the HTML looks right, but my code has blue "error" underlines
wherever I type Master.Master.PropertyName. When I hover over the
underlined code, I get the following message:

Reference required to assembly 'App_Web_appmain.maste.6044e34.4epngtbx,
Version=0.0.0.0, Culture=neutral, PublicKey Token=null' containing the type
'ASP.masters_appmain_master'. Add one to your project.

AppMain_master is my parent master page.

Can anyone tell me how to get rid of this error?
Thanks!
Diane Y.