Bundles can't contain multiple app packages for the same processor architecture Windows 10

I am upgrading (porting) an existing app from 8.1 to Windows 10. When I try to build a package I get this error:

The package with file name "MyWindows10App_1.1.0.0_x64.appx" and package full name "26637BetsySoft.Myversion81App_1.1.0.0_x86__grkkn6rpr1pew" is not valid in the bundle because the bundle also contains the package with file name "MyWindows10App_1.1.0.0_x86.appx" and package full name "26637BetsySoft.Myversion81App_1.1.0.0_x86__grkkn6rpr1pew" which applies to the same processor architecture.
Bundles can't contain multiple app packages for the same processor architecture, or an architecture-neutral app package with any architecture-specific app package.

My Windows 8.1 app was uploaded as an appxBundle and for that reason VS says it will create a bundle by default. Does anybody have any suggestions? I have tried changing the 'platform target' and configuration for the build (several different configurations). I can't get past the error.

additional messages:

• Bundle creation failed. C:\WS_VS2015\WalkScore_W10\WalkScore_W10\MakeAppx
• Error 0x80080204 - The specified package format is not valid: The package manifest is not valid.