diff --git a/composer.json b/composer.json
index 47ceed0..871d732 100644
--- a/composer.json
+++ b/composer.json
@@ -20,12 +20,14 @@
   "require": {
     "php": "~7.4||~8.0",
     "ext-curl": "*",
+    "ext-json": "*",
     "guzzlehttp/guzzle": "~6.1|~7.0",
     "composer/ca-bundle": "^1.5"
   },
   "require-dev": {
     "phpunit/phpunit": "9.5.10",
-    "squizlabs/php_codesniffer": "3.6.1"
+    "squizlabs/php_codesniffer": "3.6.1",
+    "phpstan/phpstan": "^1.11"
   },
   "autoload": {
     "psr-4": {
@@ -34,10 +36,12 @@
   },
   "scripts": {
     "ci": [
+      "@phpstan",
       "@phpcs",
       "@test"
     ],
     "test": "phpunit",
-    "phpcs": "phpcs --standard=PSR2 src && phpcs --standard=PSR2 tests"
+    "phpcs": "phpcs --standard=PSR2 src && phpcs --standard=PSR2 tests",
+    "phpstan": "phpstan analyse -l 8 src tests"
   }
 }
diff --git a/src/FioApi/Downloader.php b/src/FioApi/Downloader.php
index e96221f..9964ec9 100644
--- a/src/FioApi/Downloader.php
+++ b/src/FioApi/Downloader.php
@@ -8,6 +8,8 @@
 use FioApi\Exceptions\TooGreedyException;
 use GuzzleHttp\Client;
 use GuzzleHttp\ClientInterface;
+use GuzzleHttp\Exception\BadResponseException;
+use GuzzleHttp\Exception\GuzzleException;
 use GuzzleHttp\RequestOptions;
 use Psr\Http\Message\ResponseInterface;
 
@@ -16,7 +18,7 @@ class Downloader
     /** @var UrlBuilder */
     protected $urlBuilder;
 
-    /** @var Client */
+    /** @var ?ClientInterface */
     protected $client;
 
     public function __construct(string $token, ClientInterface $client = null)
@@ -27,7 +29,7 @@ public function __construct(string $token, ClientInterface $client = null)
 
     public function getClient(): ClientInterface
     {
-        if (!$this->client) {
+        if ($this->client === null) {
             $this->client = new Client([
                 RequestOptions::VERIFY => CaBundle::getSystemCaRootBundlePath()
             ]);
@@ -59,26 +61,34 @@ public function setLastId(string $id): void
 
         try {
             $client->request('get', $url);
-        } catch (\GuzzleHttp\Exception\BadResponseException $e) {
+        } catch (BadResponseException $e) {
             $this->handleException($e);
         }
     }
 
+    /**
+     * @param string $url
+     * @return TransactionList
+     */
     private function downloadTransactionsList(string $url): TransactionList
     {
         $client = $this->getClient();
+        $transactions = null;
 
         try {
-            /** @var ResponseInterface $response */
+            /** @var ?ResponseInterface $response */
             $response = $client->request('get', $url);
-        } catch (\GuzzleHttp\Exception\BadResponseException $e) {
+            if ($response) {
+                $transactions = json_decode($response->getBody()->getContents())->accountStatement;
+            }
+        } catch (BadResponseException $e) {
             $this->handleException($e);
         }
 
-        return TransactionList::create(json_decode($response->getBody()->getContents())->accountStatement);
+        return TransactionList::create($transactions);
     }
 
-    private function handleException(\GuzzleHttp\Exception\BadResponseException $e): void
+    private function handleException(BadResponseException $e): void
     {
         if ($e->getCode() == 409) {
             throw new TooGreedyException('You can use one token for API call every 30 seconds', $e->getCode(), $e);
diff --git a/src/FioApi/TransactionList.php b/src/FioApi/TransactionList.php
index 5ed1e94..1ad827d 100644
--- a/src/FioApi/TransactionList.php
+++ b/src/FioApi/TransactionList.php
@@ -17,13 +17,13 @@ class TransactionList
     /** @var \DateTimeImmutable */
     protected $dateEnd;
 
-    /** @var float */
+    /** @var float|null */
     protected $idFrom;
 
-    /** @var float */
+    /** @var float|null */
     protected $idTo;
 
-    /** @var int */
+    /** @var int|null */
     protected $idLastDownload;
 
     /** @var Account */
@@ -55,7 +55,7 @@ protected function __construct(
     /**
      * @param Transaction $transaction
      */
-    protected function addTransaction(Transaction $transaction)
+    protected function addTransaction(Transaction $transaction): void
     {
         $this->transactions[] = $transaction;
     }
diff --git a/src/FioApi/UrlBuilder.php b/src/FioApi/UrlBuilder.php
index 3e4d9cc..666a7eb 100644
--- a/src/FioApi/UrlBuilder.php
+++ b/src/FioApi/UrlBuilder.php
@@ -22,7 +22,7 @@ public function getToken(): string
         return $this->token;
     }
 
-    public function setToken(string $token)
+    public function setToken(string $token): void
     {
         if (!$token) {
             throw new MissingTokenException(
diff --git a/tests/FioApi/AccountTest.php b/tests/FioApi/AccountTest.php
index d9a257a..2dc810c 100644
--- a/tests/FioApi/AccountTest.php
+++ b/tests/FioApi/AccountTest.php
@@ -5,7 +5,7 @@
 
 class AccountTest extends \PHPUnit\Framework\TestCase
 {
-    public function testAccountValuesAreProperlySet()
+    public function testAccountValuesAreProperlySet(): void
     {
         $account = new Account(
             '214498596',
diff --git a/tests/FioApi/DownloaderTest.php b/tests/FioApi/DownloaderTest.php
index 33949c7..6879019 100644
--- a/tests/FioApi/DownloaderTest.php
+++ b/tests/FioApi/DownloaderTest.php
@@ -11,7 +11,7 @@
 
 class DownloaderTest extends \PHPUnit\Framework\TestCase
 {
-    public function testNotRespectingTheTimeoutResultsInTooGreedyException()
+    public function testNotRespectingTheTimeoutResultsInTooGreedyException(): void
     {
         $handler = HandlerStack::create(new MockHandler([
             new Response(409),
@@ -23,7 +23,7 @@ public function testNotRespectingTheTimeoutResultsInTooGreedyException()
         $downloader->downloadSince(new \DateTimeImmutable('-1 week'));
     }
 
-    public function testInvalidTokenResultsInInternalErrorException()
+    public function testInvalidTokenResultsInInternalErrorException(): void
     {
         $handler = HandlerStack::create(new MockHandler([
             new Response(500),
@@ -35,7 +35,7 @@ public function testInvalidTokenResultsInInternalErrorException()
         $downloader->downloadSince(new \DateTimeImmutable('-1 week'));
     }
 
-    public function testUnknownResponseCodePassesOriginalException()
+    public function testUnknownResponseCodePassesOriginalException(): void
     {
         $handler = HandlerStack::create(new MockHandler([
             new Response(418),
@@ -48,10 +48,10 @@ public function testUnknownResponseCodePassesOriginalException()
         $downloader->downloadSince(new \DateTimeImmutable('-1 week'));
     }
 
-    public function testDownloaderDownloadsData()
+    public function testDownloaderDownloadsData(): void
     {
         $handler = HandlerStack::create(new MockHandler([
-            new Response(200, [], file_get_contents(__DIR__ . '/data/example-response.json')),
+            new Response(200, [], (string) file_get_contents(__DIR__ . '/data/example-response.json')),
         ]));
         $downloader = new Downloader('validToken', new Client(['handler' => $handler]));
 
@@ -60,10 +60,10 @@ public function testDownloaderDownloadsData()
         $this->assertInstanceOf(TransactionList::class, $transactionList);
     }
 
-    public function testDownloaderDownloadsLast()
+    public function testDownloaderDownloadsLast(): void
     {
         $handler = HandlerStack::create(new MockHandler([
-            new Response(200, [], file_get_contents(__DIR__ . '/data/example-response.json')),
+            new Response(200, [], (string) file_get_contents(__DIR__ . '/data/example-response.json')),
         ]));
         $downloader = new Downloader('validToken', new Client(['handler' => $handler]));
 
@@ -72,7 +72,7 @@ public function testDownloaderDownloadsLast()
         $this->assertInstanceOf(TransactionList::class, $transactionList);
     }
 
-    public function testDownloaderSetsLastId()
+    public function testDownloaderSetsLastId(): void
     {
         $container = [];
         $history = Middleware::history($container);
diff --git a/tests/FioApi/TransactionListTest.php b/tests/FioApi/TransactionListTest.php
index 39b817b..8a3786a 100644
--- a/tests/FioApi/TransactionListTest.php
+++ b/tests/FioApi/TransactionListTest.php
@@ -5,9 +5,9 @@
 
 class TransactionListTest extends \PHPUnit\Framework\TestCase
 {
-    public function testTransactionListValuesAreProperlySet()
+    public function testTransactionListValuesAreProperlySet(): void
     {
-        $transactionList = json_decode(file_get_contents(__DIR__ . '/data/example-response.json'));
+        $transactionList = json_decode((string) file_get_contents(__DIR__ . '/data/example-response.json'));
 
         $transactionList = TransactionList::create($transactionList->accountStatement);
 
@@ -22,9 +22,9 @@ public function testTransactionListValuesAreProperlySet()
         $this->assertInstanceOf(Transaction::class, $transactionList->getTransactions()[0]);
     }
 
-    public function testEmptyTransactionList()
+    public function testEmptyTransactionList(): void
     {
-        $transactionList = json_decode(file_get_contents(__DIR__ . '/data/example-empty-response.json'));
+        $transactionList = json_decode((string) file_get_contents(__DIR__ . '/data/example-empty-response.json'));
 
         $transactionList = TransactionList::create($transactionList->accountStatement);
 
diff --git a/tests/FioApi/TransactionTest.php b/tests/FioApi/TransactionTest.php
index f32605a..8533c63 100644
--- a/tests/FioApi/TransactionTest.php
+++ b/tests/FioApi/TransactionTest.php
@@ -5,9 +5,9 @@
 
 class TransactionTest extends \PHPUnit\Framework\TestCase
 {
-    public function testAccountValuesAreProperlySet()
+    public function testAccountValuesAreProperlySet(): void
     {
-        $transaction = json_decode(file_get_contents(__DIR__ . '/data/example-transaction.json'));
+        $transaction = json_decode((string) file_get_contents(__DIR__ . '/data/example-transaction.json'));
 
         $transaction = Transaction::create($transaction);
 
diff --git a/tests/FioApi/UrlBuilderTest.php b/tests/FioApi/UrlBuilderTest.php
index bc5c788..ba33c96 100644
--- a/tests/FioApi/UrlBuilderTest.php
+++ b/tests/FioApi/UrlBuilderTest.php
@@ -5,27 +5,27 @@
 
 class UrlBuilderTest extends \PHPUnit\Framework\TestCase
 {
-    public function testMissingTokenExceptionIsThrownForEmptyToken()
+    public function testMissingTokenExceptionIsThrownForEmptyToken(): void
     {
         $this->expectException(\FioApi\Exceptions\MissingTokenException::class);
 
         new UrlBuilder('');
     }
 
-    public function testTokenCanBeSetThroughConstructor()
+    public function testTokenCanBeSetThroughConstructor(): void
     {
         $urlBuilder = new UrlBuilder('token1');
         $this->assertSame('token1', $urlBuilder->getToken());
     }
 
-    public function testTokenCanBeChangedThroughSetter()
+    public function testTokenCanBeChangedThroughSetter(): void
     {
         $urlBuilder = new UrlBuilder('token1');
         $urlBuilder->setToken('token2');
         $this->assertSame('token2', $urlBuilder->getToken());
     }
 
-    public function testBuildPeriodsUrlReturnValidUrl()
+    public function testBuildPeriodsUrlReturnValidUrl(): void
     {
         $urlBuilder = new UrlBuilder('token1');
         $this->assertSame(