The INSERT statement conflicted with the FOREIGN KEY??

Question

Keep getting the error:  The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Threads_Users".   table "dbo.Users", column 'uid'.

Users table:

CREATE TABLE [dbo].[Users] (

    [uid]       INT           IDENTITY (1, 1) NOT NULL,

    [username]  VARCHAR (14)  NOT NULL,

    [firstName] VARCHAR (20)  NULL,

    [lastName]  VARCHAR (20)  NULL,

    [gender]    VARCHAR (6)   NULL,

    [picture]   VARCHAR (MAX) NULL,

    CONSTRAINT [PK_Users] PRIMARY KEY CLUSTERED ([uid] ASC)
);

Threads table:

CREATE TABLE [dbo].[Threads] (

    [ThreadId]   INT          IDENTITY (1, 1) NOT NULL,

    [TopicId]    INT          NULL,

    [Title]      VARCHAR (50) NULL,

    [Content]    NTEXT        NULL,

    [PostedBy]   VARCHAR (30) NULL,

    [PostedDate] DATETIME     NULL,

    [uid]        INT          NULL,

    CONSTRAINT [PK_Threads] PRIMARY KEY CLUSTERED ([ThreadId] ASC),

    CONSTRAINT [FK_Threads_Users] FOREIGN KEY ([uid]) REFERENCES [dbo].[Users] ([uid]),

CONSTRAINT [FK_Threads_Threads] FOREIGN KEY ([TopicId]) REFERENCES [dbo].[Topics] ([TopicId])

);

SqlConnection conn;

            SqlCommand comm;

            conn = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename=" + Server.MapPath("~\\App_Data\\ForumDB.mdf") + ";Integrated Security=True;User Instance=True");

            conn.Open();

            comm = new SqlCommand("INSERT INTO Threads values(" + Convert.ToInt32(Session["tid"].ToString()) + ",'" + Subject.Text + "','" + Post.Text + "','" + Session["uname"].ToString() + "','" + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + "'," + Convert.ToInt32(Session["uid"]) + ")", conn);

            int rows = comm.ExecuteNonQuery();

            if (rows > 0)

            {

                Response.Redirect("threads.aspx?TopicId=" + Session["tid"].ToString());

            }

            conn.Close();