GridviewSort Expression syntax error

Please help me fix my code - getting a syntax error massage:
 private string GridViewSortExpression         {             get { return ViewState["SortExpression"] == null ? string.Empty : ViewState["SortExpression"]; }             set { ViewState["SortExpression"] = value; }         }

Error Message:
Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a cast?)