ASP.Net Error: The INSERT statement conflicted with the FOREIGN KEY constraint fk_UserId using VB.Net

Thank you to many great programmers in this group who are helping to get the Poll/Survey application to this far:

Questions chain:

http://www.aspforums.net/Threads/196837/ASPNet-Error-Response-is-not-available-in-this-context/

http://www.aspforums.net/Threads/203805/Poll-survey---Not-allow-the-same-person-to-vote-twice-o/

 I ran into the Insert conflict error "The INSERT statement conflicted with the FOREIGN KEY constraint "fk_UserId". The conflict occurred in database "Demo_Voting", table "dbo.Users", column 'UserId'.The statement has been terminated." 

It's poiting :

It’s pointing this line:

Line 65:         cmd.ExecuteNonQuery() 

From Protected Sub OnOptionChanged function 

Protected Sub OnOptionChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim con As SqlConnection = New SqlConnection(constr)
    Dim cmd As SqlCommand = New SqlCommand("SavePollAnswer", con)
    cmd.CommandType = CommandType.StoredProcedure
    cmd.Parameters.AddWithValue("@PollId", 1)
    cmd.Parameters.AddWithValue("@PollAnswer", rblPollOptions.SelectedValue)
    cmd.Parameters.AddWithValue("@UserId", CType(Me.Session("UserDetails"), User).UserId)
    con.Open()
    cmd.ExecuteNonQuery()
    con.Close()
    Response.Redirect(Request.Url.AbsoluteUri)
End Sub

Can you help to see why SavePollAnswer stored procedure caused the conflict?  Thanks again for great help!