OutOfMemoryException while using CryptoStream with large Strings in VB.NET

Question

my program runs into a OutOfMemoryException when I use large Strings with the following method.

        Public Function EncryptData(ByVal plaintext As String) As String
            Dim plaintextBytes() As Byte = System.Text.Encoding.Unicode.GetBytes(plaintext)
            Dim ms As New System.IO.MemoryStream
            Dim encStream As New CryptoStream(ms, TripleDes.CreateEncryptor(), System.Security.Cryptography.CryptoStreamMode.Write)
            encStream.Write(plaintextBytes, 0, plaintextBytes.Length)
            encStream.FlushFinalBlock()
            Return Convert.ToBase64String(ms.ToArray)
        End Function

The following Line is causing the exception:

encStream.Write(plaintextBytes, 0, plaintextBytes.Length)

How can I fix this problem? My method works fine with short strings.