"Procedure call or argument is not valid" error

Since this is my first post, be sure to use lots of lube ok?

I have a "working" program for a Vigenère cipher, but I am having an issue that I can't pinpoint. Anytime an accent mark is used in the plain text i get a "Procedure call or argument is not valid" error. So trying to encrypt "Vigenère cipher" for example will not work.

Using the plaintext "f" with the keyword set as "b" the output is "È". Decryption handles these characters just fine. The following code is my encryption sub:

Code:

---

    Private Sub btnEncrypt_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnEncrypt.Click

        Dim Key As String = txtKey.Text 'makes the password

        Dim TestString As String 'creates string variable to store encryptable material
        TestString = txtWord.Text

        If Len(Key) <= Len(TestString) Then 'this is run if the key is shorter than the encryptable material
            For i = 0 To (TestString.Length - Key.Length)
                'concatenates each character of the key string to itself until its length equals TestString length
                Key = String.Concat(Key, Key.Substring(i, 1))
            Next i
        End If
        'this loop is where the substitution is done
        For i = 1 To TestString.Length
            'convert string characters to character code equivalent and then the values are assigned to TestString
            Mid(TestString, i, 1) = Chr(Asc(Mid(TestString, i, 1)) + Asc(Key.Substring(i, 1)))
        Next i

        txtResult.Text = TestString
    End Sub

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The decryption part is just the same except the for loop:

Code:

---

        For i = 1 To TestString.Length
            Mid(TestString, i, 1) = Chr(Asc(Mid(TestString, i, 1)) - Asc(Key.Substring(i, 1)))
        Next i

---

I hope there is just something small, but us new guys never have that much luck! Thanks in advance.

I guess I should also say that the reason I want to be able to do have these special characters work on the encryption side is so that I could run it through multiple times with multiple key words. Thanks!

Moved To VB.NET

I did figure it out on my own if anyone here cares enough about this topic. Turned out to be a stupid error as per usual. It was an out of range issue. I needed to use ChrW() and AscW() in my encryption and decryption subs to get the character codes to be recognized as valid.