padding is invalid and cannot be removed

private static string DecryptWithAES(string encryptedData, Tuple<byte[],> encryptionKeys, int keySize)
       {
           string decryptedData = String.Empty;
           ICryptoTransform aesDecCryptoTransform = null;
           MemoryStream decMemStream = null;
           CryptoStream decCryptoStream = null;
           StreamReader decSr = null;

           try
           {
               using (AesManaged l_aesManaged = new AesManaged())
               {
                   l_aesManaged.KeySize = keySize;
                   l_aesManaged.BlockSize = 128;
                   l_aesManaged.Mode = CipherMode.CBC;
                   l_aesManaged.Padding = PaddingMode.PKCS7;
                   //Create decryptor crypto transform object using the hashed keys
                   aesDecCryptoTransform = l_aesManaged.CreateDecryptor(encryptionKeys.Item1, encryptionKeys.Item2);

                   byte[] originalEncData = Convert.FromBase64String(encryptedData);
                   //Check for original encrypted value length. this is to validate the input data
                   if (originalEncData.Length % aesDecCryptoTransform.InputBlockSize != 0)
                       return decryptedData;
                   //Write the encrypted value to memory stream
                   decMemStream = new MemoryStream(originalEncData, 0, originalEncData.Length);
                   decCryptoStream = new CryptoStream(decMemStream, aesDecCryptoTransform, CryptoStreamMode.Read);
                   decSr = new StreamReader(decCryptoStream);
                   decryptedData = decSr.ReadToEnd();
               }

           }
           catch { }
           finally
           {
               if (aesDecCryptoTransform != null) aesDecCryptoTransform.Dispose();
               if (decMemStream != null) decMemStream.Dispose();
           }

           return decryptedData;
       }