for AddMvcCore().AddJsonFormatters(); System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

The Startup for my Web API works on my development machine, but after a recent deployment unexpectedly stopped working on my IIS staging webserver (and previous deployments it did work).

I get this exception;

Application startup exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.ComputeClassification(String dependency)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultAssemblyPartDiscoveryProvider.CandidateResolver.<GetCandidates>d__4.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__17`2.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.GetApplicationPartManager(IServiceCollection services)
   at Microsoft.Extensions.DependencyInjection.MvcCoreServiceCollectionExtensions.AddMvcCore(IServiceCollection services)
   at Properties.API.Startup.ConfigureServices(IServiceCollection services)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()
Hosting environment: Staging

And the code is;

public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors();
            services.AddMvcCore().AddJsonFormatters();
            services.Configure<IISOptions>(options => new IISOptions
            {
                AutomaticAuthentication = true,
                ForwardClientCertificate = false,
                ForwardWindowsAuthentication = false
            });

            var connectionStringMSurveyV2 = Configuration.GetConnectionString("MSurveyV2Db");
            services.AddScoped<MSurveyV2Db>(_ => new MSurveyV2Db(connectionStringMSurveyV2));
            var connectionStringSir = Configuration.GetConnectionString("SirDb");
            services.AddScoped<SirDb>(_ => new SirDb(connectionStringSir));
            services.AddScoped<IPropertiesRepo, PropertiesRepo>();
            services.AddScoped<ISirUoW, SirUoW>();
            services.AddScoped<IPropertyUoW, PropertyUoW>();
            services.AddScoped<Services.IMailService, Services.MailService>();
        }

So what is going on here? Should I be looking at my JSON files - which I have done? How do I find out what key is causing the problem?