.net – ASPNETCOMPILER error ASPCONFIG: Could not load file or assembly ‘Microsoft.VisualBasic.Activities.Compiler’ or one of its dependencies

I just upgraded my project to Asp.Net 4, from 3.5. When the build kicks off from TeamCity, I get the following error:

[Project "Website.metaproj" (Rebuild target(s)):] C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe -v /Website -p Website\ -u -f PrecompiledWeb\Website\
[12:11:50]: [Project "Website.metaproj" (Rebuild target(s)):] ASPNETCOMPILER error ASPCONFIG: Could not load file or assembly 'Microsoft.VisualBasic.Activities.Compiler' or one of its dependencies. An attempt was made to load a program with an incorrect format.
[12:11:50]: MSBuild output:
[12:11:50]: Copying file from "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Dependencies\wnvxls.dll" to "Website\\Bin\wnvxls.dll".
[12:11:50]: Copying file from "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Dependencies\wnvxls.xml" to "Website\\Bin\wnvxls.xml".
[12:11:50]: C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe -v /Website -p Website\ -u -f PrecompiledWeb\Website\
[12:11:50]: ASPNETCOMPILER : error ASPCONFIG: Could not load file or assembly 'Microsoft.VisualBasic.Activities.Compiler' or one of its dependencies. An attempt was made to load a program with an incorrect format. [C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Website.metaproj]
[12:11:50]: Done Building Project "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Website.metaproj" (Rebuild target(s)) -- FAILED.
[12:11:50]: Done Building Project "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln" (Rebuild target(s)) -- FAILED.
[12:11:50]: Done Building Project "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln.teamcity.patch.tcprojx" (TeamCity_Generated_Build;TeamCity_Generated_NUnitTests target(s)) -- FAILED.
[12:11:50]: Build FAILED.
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln.teamcity.patch.tcprojx" (TeamCity_Generated_Build;TeamCity_Generated_NUnitTests target) (1) ->
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln" (Rebuild target) (2) ->
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Website.metaproj" (Rebuild target) (3) ->
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.Other\MyProject.Other.csproj" (Rebuild target) (5) ->
[12:11:50]: (CoreCompile target) ->
[12:11:50]: Helpers\ProgramHelper.cs(40,21): warning CS0168: The variable 'ex' is declared but never used [C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.Other\MyProject.Other.csproj]
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln.teamcity.patch.tcprojx" (TeamCity_Generated_Build;TeamCity_Generated_NUnitTests target) (1) ->
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\MyProject.sln" (Rebuild target) (2) ->
[12:11:50]: "C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Website.metaproj" (Rebuild target) (3) ->
[12:11:50]: (Rebuild target) ->
[12:11:50]: ASPNETCOMPILER : error ASPCONFIG: Could not load file or assembly 'Microsoft.VisualBasic.Activities.Compiler' or one of its dependencies. An attempt was made to load a program with an incorrect format. [C:\Program Files\TeamCity\buildAgent\work\8bbb8fc03bd91944\Website.metaproj]
[12:11:50]: 1 Warning(s)
[12:11:50]: 1 Error(s)
[12:11:50]: Time Elapsed 00:00:31.48

I tried copying the .net framework reference assemblies to C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0 thinking that would fix things, but still no luck. Any thoughts?