Unable to cast object

Unable to cast object of type 'System.Web.Mvc.RedirectToRouteResult' to type 'System.Web.Mvc.JsonResult'.

[Fact]
     public async void FindContractDetailsAsync_ByContractNo()
     {
         // Arrange
         Mock.Arrange(() => this.queryService.ExecuteAsync(Arg.IsAny<GetByIdQueryAsyc<Contract>>())).Returns(Task.FromResult(this.GetContract()));
         ContractController contractController = this.ContractController;

         // Act
         ActionResult result = await contractController.FindContractDetailsAsync();

         // Assert
         Assert.NotNull(result);
         Assert.Equal(expected: "ContractDetails", actual: ((ViewResult)result).ViewName);
         Assert.IsType(typeof(ViewResult), (ViewResult)result);
     }

[ActionName("ContractDetail")]
      public async Task<ActionResult> FindContractDetailsAsync()
      {
          try
          {
              ContractDetailsViewModel contractDetailsViewModel = default(ContractDetailsViewModel);
              var navigationViewModel = (NavigationViewModel)this.TempData["NavigationViewModel"];
              if (navigationViewModel != null)
              {
                  navigationViewModel = (NavigationViewModel)this.TempData["NavigationViewModel"];
                  contractDetailsViewModel = await this.ReadContractDetailsByContractNoAsync(navigationViewModel.Id).ConfigureAwait(continueOnCapturedContext: false);
              }

              if (contractDetailsViewModel == null)
              {
                  return this.RedirectToPageNotFound();
              }

              if (navigationViewModel.IsSuccess)
              {
                  contractDetailsViewModel.IsMessageDisplayed = navigationViewModel.IsMessageDisplayed;
                  contractDetailsViewModel.IsEdit = navigationViewModel.IsEdit;
                  navigationViewModel.IsMessageDisplayed = true;
              }

              this.TempData["NavigationViewModel"] = navigationViewModel;
              this.TempData.Keep();
              return this.View(viewName: "ContractDetails", model: contractDetailsViewModel);
          }
          catch (Exception ex)
          {
              return this.RedirectToAction(actionName: "Error", controllerName: "Error");
          }
      }