Could not load type ' xx ' from assembly because the method ' yy ' has no implementation (no RVA)

I am trying to merge two DLL with ilmerge in VS2015 but I am receiving this error when registering the merged DLL with the 2016 plugin registration tool. Here is the error log ... I tried removing the DLL's, and downgrading the .net version on the assemblies but no luck.

Unhandled Exception: System.TypeLoadException: Could not load type ' xx' from assembly 'Account.Plugin.Merged, Version=1.0.0.0, Culture=neutral, PublicKeyToken=48f4ec8800e56f69' because the method 'GrantAccess' has no implementation (no RVA).
   at System.Reflection.RuntimeAssembly.GetExportedTypes(RuntimeAssembly assembly, ObjectHandleOnStack retTypes)
   at System.Reflection.RuntimeAssembly.GetExportedTypes()
   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()

Environments: CRM2016, On-premise, Update 0.1, VS 2015 - Update 2 and .net version 4.6.1