ViewModel "no key defined" error

I've recently been learning asp.net MVC 5 and EF6 and i've started to develop viewmodels for my views instead of stuffing values into the viewbag.

I this is my viewmodel called EmployeeCreate

namespace Rota.ViewModels
{
    public class EmployeeCreate
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public IEnumerable<SelectListItem> Departments { get; set; }
    }
}

what i would like to do is on the create [httpget] action method to be able to populate a dropdown list

this is my Employee enitity

namespace Rota.Models
{
    public class Employee 
    {
        public int ID { get; set; }

        [Required]
        [Display(Name = "First Name")]
        public string FirstName { get; set; }

        [Required]
        [Display(Name = "Last Name")]
        public string LastName { get; set; }

        [ForeignKey("Department")]
        [DisplayFormat(NullDisplayText="No Department")]
        public int? DepartmentID { get; set; }

        public virtual Department Department { get; set; }
        public virtual ICollection<Schedule> Schedule { get; set; }

    }
}

and this is my Department entity

    namespace Rota.Models
    {
        public class Department
        {
            [DatabaseGenerated(DatabaseGeneratedOption.None)]
            public int ID { get; set; }

            [Required]
            public string Name { get; set; }

            public virtual ICollection<Employee> Employees { get; set; }
        }
    }

Edit this is my DbContext class

public class RotaContext : DbContext
    {
        public RotaContext() : base("RotaContext")
        {

        }

        public DbSet<Department> Departments { get; set; }
        public DbSet<Shift> Shifts { get; set; }
        public DbSet<Employee> Employees { get; set; }
        public DbSet<Schedule> Scheduled { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        }

        public System.Data.Entity.DbSet<Rota.ViewModels.EmployeeCreate> CreateEmployeeViewModels { get; set; }

    }

this is my Create Action method in my controller

namespace Rota.Controllers
{
    public class EmployeeController : Controller
    {
        //establish database connection
        private RotaContext db = new RotaContext();

        //GET: Employee/Create
        [HttpGet]
        public ActionResult Create()
        {
            EmployeeCreate ViewModel = new EmployeeCreate()
            {
                Departments = db.Departments.Select(department => new SelectListItem {
                    Value = department.ID.ToString(),
                    Text = department.Name
                })
            };
            return View(ViewModel);
        }
    }
}

The trouble i've got is that i keep getting this error :

One or more validation errors were detected during model generation:

Rota.Entities.EmployeeCreate: : EntityType 'EmployeeCreate' has no key defined. Define the key for this EntityType. CreateEmployeeViewModels: EntityType: EntitySet 'CreateEmployeeViewModels' is based on type 'EmployeeCreate' that has no keys defined.

I have no idea why the application thinks my viewmodel is an entity, i can't see anything here that might suggest as such, to my knowledge, i thought i was populating an object with values from a database, and becuase the viewmodel has absolutely no association with the entities other than this, it doesn't need a key, because it doesn't need a unique identifier.

Full Stack Trace

[ModelValidationException: One or more validation errors were detected during model generation:

Rota.Entities.EmployeeCreate: : EntityType 'EmployeeCreate' has no key defined. Define the key for this EntityType. CreateEmployeeViewModels: EntityType: EntitySet 'CreateEmployeeViewModels' is based on type 'EmployeeCreate' that has no keys defined. ]
System.Data.Entity.Core.Metadata.Edm.EdmModel.Validate() +338
System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo) +370
System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection) +288
System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext) +94
System.Data.Entity.Internal.RetryLazy2.GetValue(TInput input) +248
System.Data.Entity.Internal.LazyInternalContext.InitializeContext() +543 System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType) +26
System.Data.Entity.Internal.Linq.InternalSet1.Initialize() +72
System.Data.Entity.Internal.Linq.InternalSet1.get_InternalContext() +21 System.Data.Entity.Infrastructure.DbQuery1.System.Linq.IQueryable.get_Provider() +64 System.Linq.Queryable.Select(IQueryable1 source, Expression1 selector) +85 Rota.Controllers.EmployeeController.Create() in d:\Development\Rota\Rota\Controllers\EmployeeController.cs:87
lambda_method(Closure , ControllerBase , Object[] ) +79
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary2 parameters) +242
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary2 parameters) +39
System.Web.Mvc.Async.AsyncControllerActionInvoker.b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +12
System.Web.Mvc.Async.WrappedAsyncResult2.CallEndDelegate(IAsyncResult asyncResult) +139
System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +112 System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +452 System.Web.Mvc.Async.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult) +15
System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +37 System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +241
System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +29
System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +111
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +53 System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +19
System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +51
System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +111
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +606 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +288