Session variables issue with async operations in ASP.NET4.0 webform

Hi,
We are using async operations in ASP.NET4.0 webform. When we store some value in session variable from async operation and make redirect to new page, the session variables are becoming null. Did anyone experience the same issue and what is workaround?
Sample code:
private async Task SearchCustomerByID()
{
string value = txtCustomerId.Text;
string searchCustId = string.Empty;
IBusinessClientContext businessClientContext = new BusinessClientContext();
CSRClient csrClient = new CSRClient(businessClientContext);
CAFAccountSearchRequest csrDataModel = new CAFAccountSearchRequest();
if (value != null)
{
BusinessServiceResponse<CSRProfileResponse> objCAF;
csrDataModel.UserID = value;
BusinessServiceResponse<CSRProfileResponse> responseSearchCustomerId = await csrClient.SearchCustomerAsync(csrDataModel);
Session["CAFViewProfile"] = responseSearc ...