Moq Controller to ControllerBase convert error

I am getting this error when I am trying to use moq so that I can access application variables. In this case the application variable is ConnectionString with a value of TheConnectionString. I need the value to be available in the GetCompanyList() because the value is used inside accountService class

The Error is :

Error 5 Argument 3: cannot convert from 'Webapp.AccountServiceController' to 'System.Web.Mvc.ControllerBase'

[TestMethod]
public void TestGetCompanyList()
 {
     var accountController = new AccountServiceController();
     var context = new Mock<HttpContextBase>();
     var application = new Mock<HttpApplicationStateBase>();
     var request = new Mock<HttpRequestBase>();

     application.Setup(m => m.Add("ConnectionString","TheConnectionString");
     request.SetupGet(x => x.Headers).Returns(new System.Net.WebHeaderCollection { { "X-Requested-With", "XMLHttpRequest" } });
     context.SetupGet(ctx => ctx.Request).Returns(request.Object);
     context.SetupGet(ctx => ctx.Application).Returns(application.Object);

     accountController.ControllerContext = new ControllerContext(context.Object, new RouteData(), accountController); //Error here

     CompanyInput cInput = new CompanyInput();
     cInput.IssuerName = "BroadRidge";
     cInput.Ticker = "BR";
     var result = accountController.GetCompanyList(cInput) as IEnumerable<CompanyListResult>;
     Assert.IsNotNull(result);
 }

Controller:

public class AccountServiceController : ApiController
{
    public AccountServiceFacade accoutService;
    public AccountServiceController()
    {
        accoutService = new AccountServiceFacade();
    }

    public AccountServiceController(AccountServiceFacade facade)
    {
        accoutService = facade;
    }

    [System.Web.Http.HttpPost]
    public dynamic GetCompanyList([FromBody]CompanyInput cInput)
    {
        IEnumerable<CompanyListResult> companyList = accoutService.GetCompanyList(cInput);
        return companyList;
    }
}