.NET AES Encryption and Android Decryption

I'm encrypting a GUID string with AES in .NET with the following code.

        // Decode the challenge bytes from base 64
        byte[] challengeBytes = Base64.decode(challenge, Base64.NO_WRAP);

        ICryptoTransform encryptor = aes.CreateEncryptor(key, initializationVector);

        MemoryStream memoryStream = new MemoryStream();

        // Create a stream to encrypt our data
        CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
        cryptoStream.Write(challenge, 0, challenge.Length);
        cryptoStream.FlushFinalBlock();

        byte[] encryptedChallengeBytes = memoryStream.ToArray();

        // Clean up
        memoryStream.Close();
        cryptoStream.Close();

        // Convert to a base 64 string
        return Convert.ToBase64String(encryptedChallengeBytes, Base64FormattingOptions.None);

Then in Android I'm decrypting with the following code.

    Cipher aes = Cipher.getInstance("AES/CBC/PKCS7Padding");

    SecretKey key = new SecretKeySpec(keyBytes, "AES");

    // Initialize the cipher
    aes.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(initializationVector));

    guid = new String(aes.doFinal(challengeBytes));

    return guid;

I've done all sorts of debugging and IV, the Key and Challenge Bytes are exactly the same on both platforms. I've looked at them byte by byte and they're good. They only difference is that on .NET they range from 0-255 and on Android they range from -128 to 127. Now I'm not sure if that is the issue or not but I just assume that 2's complement shouldn't affect the Java implementation of the algorithm because that would just be stupid in my opinion.

I'm getting this exception when trying to decrypt

03-01 20:31:48.313  12886-12982/com.danielwardin.social I/SOCIAL﹕ eneter.messaging.messagingsystems.simplemessagingsystembase.internal.DefaultDuplexOutputChannel@2474079b
03-01 20:32:50.157  12886-12988/com.danielwardin.social W/System.err﹕ javax.crypto.BadPaddingException: error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt
03-01 20:32:50.158  12886-12988/com.danielwardin.social W/System.err﹕ at com.android.org.conscrypt.NativeCrypto.EVP_CipherFinal_ex(Native Method)
03-01 20:32:50.187  12886-12988/com.danielwardin.social W/System.err﹕ at com.android.org.conscrypt.OpenSSLCipher.doFinalInternal(OpenSSLCipher.java:430)
03-01 20:32:50.188  12886-12988/com.danielwardin.social W/System.err﹕ at com.android.org.conscrypt.OpenSSLCipher.engineDoFinal(OpenSSLCipher.java:466)
03-01 20:32:50.188  12886-12988/com.danielwardin.social W/System.err﹕ at javax.crypto.Cipher.doFinal(Cipher.java:1340)

This is the line causing the trouble

    guid = new String(aes.doFinal(challengeBytes));

The challengebytes array is always 48 bytes which is a multiple of 16 as well so I have no idea why I'm getting the BadPaddingException.

I've looked all over Google and tweaked it for around 4 hours and my brain is just dead now.

Any ideas?