String gets corrupted.

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim T As Byte = 0                                           'Text byte.
    Dim Hold As String = ""                                     'Holding string for final output.
    Dim CryptKey As String = "DefaultCryptKey1234567890"        'CryptKey.
    Dim K As Integer = 0                                        'CryptKey byte.
    Dim KeyPosition As Integer = 1                              'Current position in the Cryptkey.
    If TextBox2.Text <> "" Then CryptKey = TextBox2.Text        'Use default CryptKey if none entered.
    For LoopCount = 1 To Len(TextBox1.Text)                     'Main loop.
        If KeyPosition >= Len(CryptKey) Then KeyPosition = 1    'If end of CryptKey is reached, go to start again.
        K = Asc(Mid(CryptKey, KeyPosition, 1))                  'Get a new cryptkey character.
        T = Asc(Mid(TextBox1.Text, LoopCount, 1))               'Get a new text character.
        Hold += Chr(T Xor K)                                    'XOR them and add to Hold string.
        KeyPosition += 1                                        'Increment the character position in CryptKey.
    Next                                                        'All characters done?
    TextBox1.Text = Hold                                        'Dump Hold string into Textbox.
End Sub