Error with membership references

The last line at Membership.GetUser.ProviderUserKey gives me an error message - 'Public Overridable ReadOnly Property ProviderUserKey As Object' is declared in project 'System.Web.ApplicationServices.dll', which is not referenced by project '5_Secure_Members.aspx'.  Add a reference to 'System.Web.ApplicationServices.dll' to your project '5_Secure_Members.aspx'.
Can someone please offer guidance?
Dim ConnectionString As String = ConfigurationManager.ConnectionStrings("LocalSqlServer").ConnectionString() Dim checkSql As String = "SELECT MembType FROM Subscriptions WHERE UserId = @UserId" Using myconnection As New SqlConnection(ConnectionString) Dim command As New SqlCommand(checkSql, myconnection) myconnection.Open() command.Parameters.AddWithValue("@UserId", Membership.GetUser.ProviderUserKey)
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