SqlException Unhandled In Custom Membership Provider??

can someone help me in this???? this function is throwing exception "sqlException unhandled in user code"."incorrect syntax near ?" 

 

 

Public Overrides Sub UpdateUser(ByVal us As System.Web.Security.MembershipUser)

Dim conn As New SqlConnection(connStr)

Dim cmd As New SqlCommand("UPDATE users SET firstname = ?, lastname = ?, company= ?, address= ?, state= ? WHERE Username = ?", conn)

Dim u As User = CType(us, User)

cmd.Parameters.Add("@firstname", SqlDbType.VarChar, 50).Value = u.FirstName

cmd.Parameters.Add("@lastname", SqlDbType.VarChar, 50).Value = u.LastName

cmd.Parameters.Add("@company", SqlDbType.VarChar, 50).Value = u.Company

cmd.Parameters.Add("@address", SqlDbType.NVarChar, 200).Value = u.Address

cmd.Parameters.Add("@state", SqlDbType.VarChar, 50).Value = u.State

cmd.Parameters.Add("@Username", SqlDbType.NVarChar, 64).Value = u.UserName

Try

conn.Open()

cmd.ExecuteNonQuery()

Catch e As SqlException

Throw e

'Throw New Exception("Some Error occured during updation...")

Finally

conn.Close()

End Try

End Sub