DllExporterNet4X64 build error

DllExporterNet4 was working fine. Using C# with using DllExporterNet4;

But I need a 64bit version so I added DllExporterNet4X64 and changed C# to using DllExporterNet4X64;

Then changed the Target to 64bit and changed the post build to DllExporterNet4X64.exe $(TargetFileName)

But when I build assembly I get an error - ildasm.exe has failed disassembling dll

Any ideas?