diff --git a/.travis.yml b/.travis.yml
index f0eada0..07da6f6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,11 @@
 language: php
 php:
   - '5.6'
+  - '7.2'
+  - '7.3'
+  - '7.4'
+  - '8.0'
 
 before_script: composer install
 
-script: phpunit --configuration tests/phpunit.xml
\ No newline at end of file
+script: phpunit --configuration tests/phpunit.xml
diff --git a/composer.json b/composer.json
index 2ffc646..33e586c 100644
--- a/composer.json
+++ b/composer.json
@@ -37,10 +37,10 @@
     "type": "library",
     "require": {
         "php": ">=5.5.9",
-        "guzzlehttp/guzzle": "~6"
+        "guzzlehttp/guzzle": "~6|~7"
     },
     "require-dev": {
-        "phpunit/phpunit": "5.7"
+        "phpunit/phpunit": ">=5.7"
     },
     "support": {
         "docs": "https://developer.avalara.com",
diff --git a/src/Client.php b/src/Client.php
index 2710b4e..aec3a52 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -54,7 +54,7 @@ class AvaTaxClientBase
      *
      * @throws \Exception
      */
-    public function __construct($appName, $appVersion, $machineName="", $environment, $guzzleParams = [])
+    public function __construct($appName, $appVersion, $machineName="", $environment=null, $guzzleParams = [])
     {
         // app name and app version are mandatory fields.
         if ($appName == "" || $appName == null || $appVersion == "" || $appVersion == null) {
diff --git a/src/ClientExtensionExample.php b/src/ClientExtensionExample.php
index f54e57e..5863362 100644
--- a/src/ClientExtensionExample.php
+++ b/src/ClientExtensionExample.php
@@ -18,7 +18,7 @@ class ClientExtensionExample extends AvaTaxClient
      * @param string $environment  Indicates which server to use; acceptable values are "sandbox" or "production", or the full URL of your AvaTax instance.
      * @param array $guzzleParams  Extra parameters to pass to the guzzle HTTP client (http://docs.guzzlephp.org/en/latest/request-options.html)
      */
-    public function __construct($appName, $appVersion, $machineName="", $environment, $guzzleParams = [], $clientConfig = null)
+    public function __construct($appName, $appVersion, $machineName="", $environment=null, $guzzleParams = [], $clientConfig = null)
     {
         parent::__construct($appName, $appVersion, $machineName, $environment, $guzzleParams);
         $this->clientConfig = $clientConfig;
diff --git a/src/Methods.php b/src/Methods.php
index 070d173..e5e1c6a 100644
--- a/src/Methods.php
+++ b/src/Methods.php
@@ -4192,7 +4192,7 @@ public function listPreferredPrograms($filter=null, $top=null, $skip=null, $orde
      * @param string $countryCode If not null, return all records with this code.
      * @return FetchResult
      */
-    public function listProductClassificationSystems($filter=null, $top=null, $skip=null, $orderBy=null, $countryCode)    {
+    public function listProductClassificationSystems($filter=null, $top=null, $skip=null, $orderBy=null, $countryCode=null)    {
         $path = "/api/v2/definitions/productclassificationsystems";
         $guzzleParams = [
             'query' => ['$filter' => $filter, '$top' => $top, '$skip' => $skip, '$orderBy' => $orderBy, '$countryCode' => $countryCode],
@@ -4218,7 +4218,7 @@ public function listProductClassificationSystems($filter=null, $top=null, $skip=
      * @param string $countryCode If not null, return all records with this code.
      * @return FetchResult
      */
-    public function listProductClassificationSystemsByCompany($companyCode, $filter=null, $top=null, $skip=null, $orderBy=null, $countryCode)    {
+    public function listProductClassificationSystemsByCompany($companyCode, $filter=null, $top=null, $skip=null, $orderBy=null, $countryCode=null)    {
         $path = "/api/v2/definitions/productclassificationsystems/bycompany/{$companyCode}";
         $guzzleParams = [
             'query' => ['$filter' => $filter, '$top' => $top, '$skip' => $skip, '$orderBy' => $orderBy, '$countryCode' => $countryCode],
@@ -6383,7 +6383,7 @@ public function validateLocation($companyId, $id)    {
      * @param AdjustMultiDocumentModel $model The adjust request you wish to execute
      * @return MultiDocumentModel
      */
-    public function adjustMultiDocumentTransaction($code, $type, $include=null, $model)    {
+    public function adjustMultiDocumentTransaction($code, $type, $include=null, $model=null)    {
         $path = "/api/v2/transactions/multidocument/{$code}/type/{$type}/adjust";
         $guzzleParams = [
             'query' => ['include' => $include],
@@ -6529,7 +6529,7 @@ public function commitMultiDocumentTransaction($model)    {
      * @param CreateMultiDocumentModel $model the multi document transaction model
      * @return MultiDocumentModel
      */
-    public function createMultiDocumentTransaction($include=null, $model)    {
+    public function createMultiDocumentTransaction($include=null, $model=null)    {
         $path = "/api/v2/transactions/multidocument";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -6747,7 +6747,7 @@ public function listMultiDocumentTransactions($filter=null, $include=null, $top=
      * @param RefundTransactionModel $model Information about the refund to create
      * @return MultiDocumentModel
      */
-    public function refundMultiDocumentTransaction($code, $type, $include=null, $model)    {
+    public function refundMultiDocumentTransaction($code, $type, $include=null, $model=null)    {
         $path = "/api/v2/transactions/multidocument/{$code}/type/{$type}/refund";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -8870,7 +8870,7 @@ public function updateTaxRule($companyId, $id, $model)    {
      * @param AddTransactionLineModel $model information about the transaction and lines to be added
      * @return TransactionModel
      */
-    public function addLines($include=null, $model)    {
+    public function addLines($include=null, $model=null)    {
         $path = "/api/v2/companies/transactions/lines/add";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -8925,7 +8925,7 @@ public function addLines($include=null, $model)    {
      * @param AdjustTransactionModel $model The adjustment you wish to make
      * @return TransactionModel
      */
-    public function adjustTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function adjustTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/adjust";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9102,7 +9102,7 @@ public function bulkLockTransaction($model)    {
      * @param ChangeTransactionCodeModel $model The code change request you wish to execute
      * @return TransactionModel
      */
-    public function changeTransactionCode($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function changeTransactionCode($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/changecode";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9155,7 +9155,7 @@ public function changeTransactionCode($companyCode, $transactionCode, $documentT
      * @param CommitTransactionModel $model The commit request you wish to execute
      * @return TransactionModel
      */
-    public function commitTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function commitTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/commit";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9211,7 +9211,7 @@ public function commitTransaction($companyCode, $transactionCode, $documentType,
      * @param CreateOrAdjustTransactionModel $model The transaction you wish to create or adjust
      * @return TransactionModel
      */
-    public function createOrAdjustTransaction($include=null, $model)    {
+    public function createOrAdjustTransaction($include=null, $model=null)    {
         $path = "/api/v2/transactions/createoradjust";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -9274,7 +9274,7 @@ public function createOrAdjustTransaction($include=null, $model)    {
      * @param CreateTransactionModel $model The transaction you wish to create
      * @return TransactionModel
      */
-    public function createTransaction($include=null, $model)    {
+    public function createTransaction($include=null, $model=null)    {
         $path = "/api/v2/transactions/create";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -9314,7 +9314,7 @@ public function createTransaction($include=null, $model)    {
      * @param RemoveTransactionLineModel $model information about the transaction and lines to be removed
      * @return TransactionModel
      */
-    public function deleteLines($include=null, $model)    {
+    public function deleteLines($include=null, $model=null)    {
         $path = "/api/v2/companies/transactions/lines/delete";
         $guzzleParams = [
             'query' => ['$include' => $include],
@@ -9551,7 +9551,7 @@ public function listTransactionsByCompany($companyCode, $dataSourceId, $include=
      * @param LockTransactionModel $model The lock request you wish to execute
      * @return TransactionModel
      */
-    public function lockTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function lockTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/lock";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9617,7 +9617,7 @@ public function lockTransaction($companyCode, $transactionCode, $documentType, $
      * @param RefundTransactionModel $model Information about the refund to create
      * @return TransactionModel
      */
-    public function refundTransaction($companyCode, $transactionCode, $include=null, $documentType, $useTaxDateOverride, $model)    {
+    public function refundTransaction($companyCode, $transactionCode, $include=null, $documentType=null, $useTaxDateOverride=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/refund";
         $guzzleParams = [
             'query' => ['$include' => $include, 'documentType' => $documentType, 'useTaxDateOverride' => $useTaxDateOverride],
@@ -9670,7 +9670,7 @@ public function refundTransaction($companyCode, $transactionCode, $include=null,
      * @param SettleTransactionModel $model The data from an external system to reconcile against AvaTax
      * @return TransactionModel
      */
-    public function settleTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function settleTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/settle";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9813,7 +9813,7 @@ public function unvoidTransaction($companyCode, $transactionCode, $documentType,
      * @param VerifyTransactionModel $model The data from an external system to reconcile against AvaTax
      * @return TransactionModel
      */
-    public function verifyTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function verifyTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/verify";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -9867,7 +9867,7 @@ public function verifyTransaction($companyCode, $transactionCode, $documentType,
      * @param VoidTransactionModel $model The void request you wish to execute. To void a transaction the code must be set to 'DocVoided'
      * @return TransactionModel
      */
-    public function voidTransaction($companyCode, $transactionCode, $documentType, $include=null, $model)    {
+    public function voidTransaction($companyCode, $transactionCode, $documentType, $include=null, $model=null)    {
         $path = "/api/v2/companies/{$companyCode}/transactions/{$transactionCode}/void";
         $guzzleParams = [
             'query' => ['documentType' => $documentType, '$include' => $include],
@@ -10384,4 +10384,4 @@ public function ping()    {
     }
 
 }
-?>
\ No newline at end of file
+?>
diff --git a/tests/basicWorkflowTest.php b/tests/basicWorkflowTest.php
index 63391fb..7180dc9 100644
--- a/tests/basicWorkflowTest.php
+++ b/tests/basicWorkflowTest.php
@@ -8,10 +8,11 @@
 final class AvaTaxClientTest extends TestCase
 {
     /**
-     * @expectedException \Exception
      */
     public function testConstructorThrowsExceptionForMissingRequirements()
     {
+        $this->expectException(Exception::class);
+
         new Avalara\AvaTaxClient('', '', '', '');
     }