Unable to debug with dll generated from plugin project build

[efei001]

When building the dll and trying to attach it to the debugger I get the following error:

Unhandled Exception: System.BadImageFormatException: Could not load file or assembly 'xxxx\bin\Debug\xxxx.Plugin.xxxx.dll' or one of its dependencies. An attempt was made to load a program with an incorrect format.
at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)
at System.Reflection.RuntimeAssembly.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark)
at System.Reflection.Assembly.LoadFrom(String assemblyFile)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.LoadAssembly(String path)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path)
at Microsoft.Crm.Tools.Libraries.CrmAssemblyReader.RetrievePluginsFromAssembly(String path)
at Microsoft.Crm.Tools.Libraries.RegistrationHelper.RetrievePluginsFromAssembly(String pathToAssembly, String appPath)
at Microsoft.Crm.Tools.AssemblyRegistration.PluginRegistrationViewModel.BtnLoadAssemblyClicked()