Build error in Windows Phone (Universal) app

I am trying to build a Windows Phone app. I am currently getting an error:

Processing Resources with error: Invalid qualifier value.
File: MakePri

and

Invalid qualifier: (null)
File: MakePRI

I was originally using the Multilingual App Toolkit, but I have since disabled it to no effect. When building with verbose output, I see (personal information redacted):

8>Using "GenerateProjectPriFile" task from assembly "C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v12.0\AppxPackage\Microsoft.Build.AppxPackage.dll".
8>Task "GenerateProjectPriFile"
8>  C:\Program Files (x86)\Windows Kits\8.1\\bin\x86\MakePri.exe New -ProjectRoot [Redacted] -ConfigXml obj\Debug\priconfig.xml -OutputFile [Redacted] -ExtensionDll "C:\Program Files (x86)\Windows Phone Kits\8.1\bin\x86\MrmEnvironmentExtDl.dll" -IndexName 14de7fbe-f3e3-43a8-a9df-dddc72828f37 -Verbose -Overwrite  
8>  Option Verbose specified
8>  Option Overwrite specified
8>  Index Pass Completed: obj\Debug\layout.resfiles
8>  Scale Qualifiers: 240
8>  
8>  Index Pass Completed: obj\Debug\resources.resfiles
8>  
8>MakePri : error 0x80073b0a: Processing Resources with error: Invalid qualifier value.
8>  
8>  
8>MakePRI : error 0xdef00042: Invalid qualifier: (null)
8>  The command exited with code -2147009782.
8>Done executing task "GenerateProjectPriFile" -- FAILED.

The output in Diagnostic mode does not give much more information.

Scaled images are only needed for the phone, so move them all to the "Assets" folder in the phone project, and single copies in the Windows "Assets" folder without qualifiers in their names. It was the only way I got the error messages to go away.