SGEN failure: An attempt was made to load an assembly with an incorrect format in VS2008

I was able to find several similar questions asked and answered, but none of the answers or exact conditions applied to my situation.

1. I have a .NET 3.5 project being built for x64.
2. I also have an x64 mixed mode reference (also targeting .NET 3.5).
3. In Visual Studio 2008, I created a Web Reference which causes SGEN to execute during a Release build to create the helper DLL.

If I don't reference the mixed-mode DLL, this works fine. If I do reference the mixed mode DLL, I get this error (xxx is just a placeholder):

SGEN : error : An attempt was made to load an assembly with an
incorrect format:
C:\code\xxx\trunk\xxx\common\xxx\build\winx64\lib\xxx.dll.

This is the command being issued:

C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin\sgen.exe
/assembly:C:\code\xxx\trunk\xxx\xxx\obj\x64\Release\xxx.dll
/proxytypes /reference:..\common\xxx\build\winx64\lib\xxx.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.configuration.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Configuration.Install.dll
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\v3.5\System.Core.dll"
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Data.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.EnterpriseServices.dll
/reference:C:\Windows\assembly\GAC_MSIL\System.Management.Automation\1.0.0.0__31bf3856ad364e35\System.Management.Automation.dll
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\v3.0\System.Runtime.Serialization.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\v3.0\System.ServiceModel.dll"
/reference:"C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\v3.5\System.Web.Extensions.dll"
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Web.Services.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Xml.dll

Any ideas?