Hash and salt

I have been playing around with a password hash and salt scheme for my application

But i keep getting an error message when it executes the command object

i get the error

Unable to cast object of type 'System.byte[]' to type 'system.inconvertible'

Any suggestions?
[code]
Sub CreateAccount(ByVal sender As Object, ByVal e As EventArgs)
Dim literror As New LiteralControl
'1. Create a connection
'Create connection string to pass database, string holds login information to mySQL,
Dim connectionString As String
connectionString = "Server=localhost; ;database=ftp1;"

'Builds .net mysql connection and passes connection string into method
Dim connection As New MySqlConnection(connectionString)
Try
'2. Create a command object for the query
Dim strSQL As String = _
"INSERT INTO Useraccount(Username,Password) " & _
"VALUES(?Username, ?Password)"
Dim objCmd As New MySqlCommand(strSQL, connection)

'3. Create parameters
Dim paramUsername As MySqlParameter
paramUsername = New MySqlParameter("?Username", SqlDbType.VarChar, 25)
paramUsername.Value = txtUsername.Text
objCmd.Parameters.Add(paramUsername)


'Encrypt the password
Dim md5Hasher As New MD5CryptoServiceProvider()

Dim hashedBytes As Byte()
Dim encoder As New UTF8Encoding()

hashedBytes = md5Hasher.ComputeHash(encoder.GetBytes(txtPwd.Text & txtusername.text))

Dim paramPwd As MySqlParameter
paramPwd = New MySqlParameter("?Password", SqlDbType.Binary, 16)
paramPwd.Value = hashedBytes
objCmd.Parameters.Add(paramPwd)


'Insert the records into the database
connection.Open()
objCmd.ExecuteReader()
connection.Close()
Catch ex As Exception
literror.Text = ex.Message
MsgBox(ex.Message)

End Try
Response.Redirect("userhome.aspx")

End Sub
[code]

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