diff --git a/src/Z38/SwissPayment/IBAN.php b/src/Z38/SwissPayment/IBAN.php
index c7472e2..ac1c13e 100644
--- a/src/Z38/SwissPayment/IBAN.php
+++ b/src/Z38/SwissPayment/IBAN.php
@@ -10,6 +10,9 @@ class IBAN
     const MAX_LENGTH = 34;
     const PATTERN = '/^[A-Z]{2,2}[0-9]{2,2}[A-Z0-9]{1,30}$/';
 
+    /**
+     * @var string
+     */
     protected $iban;
 
     /**
diff --git a/src/Z38/SwissPayment/Message/CustomerCreditTransfer.php b/src/Z38/SwissPayment/Message/CustomerCreditTransfer.php
index 79654db..fc750a4 100644
--- a/src/Z38/SwissPayment/Message/CustomerCreditTransfer.php
+++ b/src/Z38/SwissPayment/Message/CustomerCreditTransfer.php
@@ -10,9 +10,24 @@
  */
 class CustomerCreditTransfer extends AbstractMessage
 {
+    /**
+     * @var string
+     */
     protected $id;
+
+    /**
+     * @var string
+     */
     protected $initiatingParty;
+
+    /**
+     * @var array
+     */
     protected $payments;
+
+    /**
+     * @var \DateTime
+     */
     protected $creationTime;
 
     /**
@@ -23,8 +38,8 @@ class CustomerCreditTransfer extends AbstractMessage
      */
     public function __construct($id, $initiatingParty)
     {
-        $this->id = $id;
-        $this->initiatingParty = $initiatingParty;
+        $this->id = (string) $id;
+        $this->initiatingParty = (string) $initiatingParty;
         $this->payments = array();
         $this->creationTime = new \DateTime();
     }
diff --git a/src/Z38/SwissPayment/Money/Money.php b/src/Z38/SwissPayment/Money/Money.php
index dd933a5..318fedf 100644
--- a/src/Z38/SwissPayment/Money/Money.php
+++ b/src/Z38/SwissPayment/Money/Money.php
@@ -7,6 +7,9 @@
  */
 abstract class Money
 {
+    /**
+     * @var int
+     */
     protected $cents;
 
     /**
diff --git a/src/Z38/SwissPayment/PaymentInformation/PaymentInformation.php b/src/Z38/SwissPayment/PaymentInformation/PaymentInformation.php
index 9cae218..c6ec2a7 100644
--- a/src/Z38/SwissPayment/PaymentInformation/PaymentInformation.php
+++ b/src/Z38/SwissPayment/PaymentInformation/PaymentInformation.php
@@ -14,12 +14,39 @@
  */
 class PaymentInformation
 {
+    /**
+     * @var string
+     */
     protected $id;
+
+    /**
+     * @var array
+     */
     protected $transactions;
+
+    /**
+     * @var bool
+     */
     protected $batchBooking;
+
+    /**
+     * @var \DateTime
+     */
     protected $executionDate;
+
+    /**
+     * @var string
+     */
     protected $debtorName;
+
+    /**
+     * @var FinancialInstitutionInterface
+     */
     protected $debtorAgent;
+
+    /**
+     * @var IBAN
+     */
     protected $debtorIBAN;
 
     /**
@@ -36,11 +63,11 @@ public function __construct($id, $debtorName, FinancialInstitutionInterface $deb
             throw new \InvalidArgumentException('The debtor agent must be an instance of BC or BIC.');
         }
 
-        $this->id = $id;
+        $this->id = (string) $id;
         $this->transactions = array();
         $this->batchBooking = true;
         $this->executionDate = new \DateTime();
-        $this->debtorName = $debtorName;
+        $this->debtorName = (string) $debtorName;
         $this->debtorAgent = $debtorAgent;
         $this->debtorIBAN = $debtorIBAN;
     }
diff --git a/src/Z38/SwissPayment/PostalAccount.php b/src/Z38/SwissPayment/PostalAccount.php
index 153ce35..68cbeaa 100644
--- a/src/Z38/SwissPayment/PostalAccount.php
+++ b/src/Z38/SwissPayment/PostalAccount.php
@@ -9,8 +9,19 @@ class PostalAccount
 {
     const PATTERN = '/^[0-9]{2}-[1-9][0-9]{0,5}-[0-9]$/';
 
+    /**
+     * @var int
+     */
     protected $prefix;
+
+    /**
+     * @var int
+     */
     protected $number;
+
+    /**
+     * @var int
+     */
     protected $checkDigit;
 
     /**
@@ -31,9 +42,9 @@ public function __construct($postalAccount)
             throw new \InvalidArgumentException('Postal account number has an invalid prefix.');
         }
 
-        $this->prefix = $parts[0];
-        $this->number = $parts[1];
-        $this->checkDigit = $parts[2];
+        $this->prefix = (int) $parts[0];
+        $this->number = (int) $parts[1];
+        $this->checkDigit = (int) $parts[2];
     }
 
     /**
@@ -46,6 +57,13 @@ public function format()
         return sprintf('%d-%d-%d', $this->prefix, $this->number, $this->checkDigit);
     }
 
+    /**
+     * Checks whether a given prefix is valid
+     *
+     * @param int $prefix The prefix to be checked
+     *
+     * @return bool True if the prefix is valid
+     */
     private static function checkPrefix($prefix)
     {
         return in_array($prefix, array(
diff --git a/src/Z38/SwissPayment/PostalAddress.php b/src/Z38/SwissPayment/PostalAddress.php
index bb0214b..f44004f 100644
--- a/src/Z38/SwissPayment/PostalAddress.php
+++ b/src/Z38/SwissPayment/PostalAddress.php
@@ -7,10 +7,29 @@
  */
 class PostalAddress
 {
+    /**
+     * @var string
+     */
     protected $street;
+
+    /**
+     * @var string|null
+     */
     protected $buildingNo;
+
+    /**
+     * @var string
+     */
     protected $postCode;
+
+    /**
+     * @var string
+     */
     protected $town;
+
+    /**
+     * @var string
+     */
     protected $country;
 
     /**
@@ -24,11 +43,11 @@ class PostalAddress
      */
     public function __construct($street, $buildingNo, $postCode, $town, $country = 'CH')
     {
-        $this->street = $street;
+        $this->street = (string) $street;
         $this->buildingNo = $buildingNo;
-        $this->postCode = $postCode;
-        $this->town = $town;
-        $this->country = $country;
+        $this->postCode = (string) $postCode;
+        $this->town = (string) $town;
+        $this->country = (string) $country;
     }
 
     /**
diff --git a/src/Z38/SwissPayment/TransactionInformation/CreditTransfer.php b/src/Z38/SwissPayment/TransactionInformation/CreditTransfer.php
index ba5eaf1..1a50218 100644
--- a/src/Z38/SwissPayment/TransactionInformation/CreditTransfer.php
+++ b/src/Z38/SwissPayment/TransactionInformation/CreditTransfer.php
@@ -10,11 +10,34 @@
  */
 abstract class CreditTransfer
 {
+    /**
+     * @var string
+     */
     protected $instructionId;
+
+    /**
+     * @var string
+     */
     protected $endToEndId;
+
+    /**
+     * @var string
+     */
     protected $creditorName;
+
+    /**
+     * @var PostalAddress
+     */
     protected $creditorAddress;
+
+    /**
+     * @var Money
+     */
     protected $amount;
+
+    /**
+     * @var string|null
+     */
     protected $remittanceInformation;
 
     /**
@@ -28,10 +51,10 @@ abstract class CreditTransfer
      */
     public function __construct($instructionId, $endToEndId, Money $amount, $creditorName, PostalAddress $creditorAddress)
     {
-        $this->instructionId = $instructionId;
-        $this->endToEndId = $endToEndId;
+        $this->instructionId = (string) $instructionId;
+        $this->endToEndId = (string) $endToEndId;
         $this->amount = $amount;
-        $this->creditorName = $creditorName;
+        $this->creditorName = (string) $creditorName;
         $this->creditorAddress = $creditorAddress;
         $this->remittanceInformation = null;
     }
diff --git a/src/Z38/SwissPayment/TransactionInformation/IS1CreditTransfer.php b/src/Z38/SwissPayment/TransactionInformation/IS1CreditTransfer.php
index f330034..3f85064 100644
--- a/src/Z38/SwissPayment/TransactionInformation/IS1CreditTransfer.php
+++ b/src/Z38/SwissPayment/TransactionInformation/IS1CreditTransfer.php
@@ -11,6 +11,9 @@
  */
 class IS1CreditTransfer extends CreditTransfer
 {
+    /**
+     * @var PostalAccount
+     */
     protected $creditorAccount;
 
     /**
diff --git a/src/Z38/SwissPayment/TransactionInformation/IS2CreditTransfer.php b/src/Z38/SwissPayment/TransactionInformation/IS2CreditTransfer.php
index 49a1e21..3e1b091 100644
--- a/src/Z38/SwissPayment/TransactionInformation/IS2CreditTransfer.php
+++ b/src/Z38/SwissPayment/TransactionInformation/IS2CreditTransfer.php
@@ -12,8 +12,19 @@
  */
 class IS2CreditTransfer extends CreditTransfer
 {
+    /**
+     * @var IBAN
+     */
     protected $creditorIBAN;
+
+    /**
+     * @var string
+     */
     protected $creditorAgentName;
+
+    /**
+     * @var PostalAccount
+     */
     protected $creditorAgentPostal;
 
     /**
@@ -27,7 +38,7 @@ public function __construct($instructionId, $endToEndId, Money\CHF $amount, $cre
         parent::__construct($instructionId, $endToEndId, $amount, $creditorName, $creditorAddress);
 
         $this->creditorIBAN = $creditorIBAN;
-        $this->creditorAgentName = $creditorAgentName;
+        $this->creditorAgentName = (string) $creditorAgentName;
         $this->creditorAgentPostal = $creditorAgentPostal;
     }