Define a type without namespace and return it as IEnumerable and mark QueryableAttribute on it.

The exception is:

System.ArgumentNullException occurred
Message=Value cannot be null.
Parameter name: namespaceName
Source=Microsoft.Data.Edm
ParamName=namespaceName
StackTrace:

   at Microsoft.Data.Edm.EdmUtil.CheckArgumentNull[T](T value, String parameterName)
   at Microsoft.Data.Edm.Library.EdmEntityType..ctor(String namespaceName, String name, IEdmEntityType baseType, Boolean isAbstract, Boolean isOpen)
   at Microsoft.Data.Edm.Library.EdmEntityType..ctor(String namespaceName, String name)
   at System.Web.Http.OData.Builder.EdmTypeBuilder.CreateEdmTypeHeader(IStructuralTypeConfiguration config) in C:\dd\DevDiv\Offcycle\WPT\WebStackRuntime\aspnetwebstack-2df9d6d948b1\src\System.Web.Http.OData\OData\Builder\EdmTypeBuilder.cs:line 50

InnerException: