diff --git a/composer.json b/composer.json
index 90fdb87b..3d325b03 100644
--- a/composer.json
+++ b/composer.json
@@ -123,9 +123,9 @@
         ],
         "test:unit:pretty": "phpdbg -qrr vendor/bin/phpunit --coverage-html build/coverage --order-by=random",
 
-        "infect": "infection --threads=4 --min-msi=60 --log-verbosity=2 --only-covered",
-        "infect:ci": "infection --threads=4 --min-msi=60 --log-verbosity=2 --only-covered --coverage=build",
-        "infect:show": "infection --threads=4 --min-msi=60 --log-verbosity=2 --only-covered --show-mutations",
+        "infect": "infection --threads=4 --min-msi=65 --log-verbosity=2 --only-covered",
+        "infect:ci": "infection --threads=4 --min-msi=65 --log-verbosity=2 --only-covered --coverage=build",
+        "infect:show": "infection --threads=4 --min-msi=65 --log-verbosity=2 --only-covered --show-mutations",
         "infect:test": [
             "@test:unit:ci",
             "@infect:ci"
diff --git a/module/Common/src/Response/PixelResponse.php b/module/Common/src/Response/PixelResponse.php
index e2554bb5..87a10dca 100644
--- a/module/Common/src/Response/PixelResponse.php
+++ b/module/Common/src/Response/PixelResponse.php
@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Shlinkio\Shlink\Common\Response;
 
+use Fig\Http\Message\StatusCodeInterface as StatusCode;
 use Psr\Http\Message\StreamInterface;
 use Zend\Diactoros\Response;
 use Zend\Diactoros\Stream;
@@ -13,7 +14,7 @@ class PixelResponse extends Response
     private const BASE_64_IMAGE = 'R0lGODlhAQABAJAAAP8AAAAAACH5BAUQAAAALAAAAAABAAEAAAICBAEAOw==';
     private const CONTENT_TYPE = 'image/gif';
 
-    public function __construct(int $status = 200, array $headers = [])
+    public function __construct(int $status = StatusCode::STATUS_OK, array $headers = [])
     {
         $headers['content-type'] = self::CONTENT_TYPE;
         parent::__construct($this->createBody(), $status, $headers);
diff --git a/module/Common/src/Response/QrCodeResponse.php b/module/Common/src/Response/QrCodeResponse.php
index b207fa0d..230ae08f 100644
--- a/module/Common/src/Response/QrCodeResponse.php
+++ b/module/Common/src/Response/QrCodeResponse.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 namespace Shlinkio\Shlink\Common\Response;
 
 use Endroid\QrCode\QrCode;
+use Fig\Http\Message\StatusCodeInterface as StatusCode;
 use Psr\Http\Message\StreamInterface;
 use Zend\Diactoros\Response;
 use Zend\Diactoros\Stream;
@@ -12,7 +13,7 @@ class QrCodeResponse extends Response
 {
     use Response\InjectContentTypeTrait;
 
-    public function __construct(QrCode $qrCode, $status = 200, array $headers = [])
+    public function __construct(QrCode $qrCode, int $status = StatusCode::STATUS_OK, array $headers = [])
     {
         parent::__construct(
             $this->createBody($qrCode),
@@ -21,13 +22,7 @@ class QrCodeResponse extends Response
         );
     }
 
-    /**
-     * Create the message body.
-     *
-     * @param QrCode $qrCode
-     * @return StreamInterface
-     */
-    private function createBody(QrCode $qrCode)
+    private function createBody(QrCode $qrCode): StreamInterface
     {
         $body = new Stream('php://temp', 'wb+');
         $body->write($qrCode->get());
diff --git a/module/Common/src/Util/ResponseUtilsTrait.php b/module/Common/src/Util/ResponseUtilsTrait.php
index e8ad8bf0..25a884f9 100644
--- a/module/Common/src/Util/ResponseUtilsTrait.php
+++ b/module/Common/src/Util/ResponseUtilsTrait.php
@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Shlinkio\Shlink\Common\Util;
 
+use Fig\Http\Message\StatusCodeInterface as StatusCode;
 use finfo;
 use Psr\Http\Message\ResponseInterface;
 use Zend\Diactoros\Response;
@@ -20,7 +21,7 @@ trait ResponseUtilsTrait
     private function generateBinaryResponse(string $path, array $extraHeaders = []): ResponseInterface
     {
         $body = new Stream($path);
-        return new Response($body, 200, ArrayUtils::merge([
+        return new Response($body, StatusCode::STATUS_OK, ArrayUtils::merge([
             'Content-Type' => (new finfo(FILEINFO_MIME))->file($path),
             'Content-Length' => (string) $body->getSize(),
         ], $extraHeaders));
diff --git a/module/Common/src/Util/StringUtilsTrait.php b/module/Common/src/Util/StringUtilsTrait.php
index 87b77a2a..853ba7b8 100644
--- a/module/Common/src/Util/StringUtilsTrait.php
+++ b/module/Common/src/Util/StringUtilsTrait.php
@@ -9,7 +9,7 @@ use function strlen;
 
 trait StringUtilsTrait
 {
-    private function generateRandomString($length = 10): string
+    private function generateRandomString(int $length = 10): string
     {
         $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
         $charactersLength = strlen($characters);
diff --git a/module/Common/test/Response/QrCodeResponseTest.php b/module/Common/test/Response/QrCodeResponseTest.php
new file mode 100644
index 00000000..36d6fada
--- /dev/null
+++ b/module/Common/test/Response/QrCodeResponseTest.php
@@ -0,0 +1,23 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Common\Response;
+
+use Endroid\QrCode\QrCode;
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Common\Response\QrCodeResponse;
+
+class QrCodeResponseTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function providedQrCoideIsSetAsBody()
+    {
+        $qrCode = new QrCode('Hello');
+        $resp = new QrCodeResponse($qrCode);
+
+        $this->assertEquals($qrCode->getContentType(), $resp->getHeaderLine('Content-Type'));
+        $this->assertEquals($qrCode->get(), (string) $resp->getBody());
+    }
+}
diff --git a/module/Common/test/Util/DateRangeTest.php b/module/Common/test/Util/DateRangeTest.php
index e5e48d46..ec5602a2 100644
--- a/module/Common/test/Util/DateRangeTest.php
+++ b/module/Common/test/Util/DateRangeTest.php
@@ -32,4 +32,24 @@ class DateRangeTest extends TestCase
         $this->assertSame($endDate, $range->getEndDate());
         $this->assertFalse($range->isEmpty());
     }
+
+    /**
+     * @test
+     * @dataProvider provideDates
+     */
+    public function isConsideredEmptyOnlyIfNoneOfTheDatesIsSet(?Chronos $startDate, ?Chronos $endDate, bool $isEmpty)
+    {
+        $range = new DateRange($startDate, $endDate);
+        $this->assertEquals($isEmpty, $range->isEmpty());
+    }
+
+    public function provideDates(): array
+    {
+        return [
+            [null, null, true],
+            [null, Chronos::now(), false],
+            [Chronos::now(), null, false],
+            [Chronos::now(), Chronos::now(), false],
+        ];
+    }
 }
diff --git a/module/Common/test/Util/StringUtilsTraitTest.php b/module/Common/test/Util/StringUtilsTraitTest.php
new file mode 100644
index 00000000..b64f4738
--- /dev/null
+++ b/module/Common/test/Util/StringUtilsTraitTest.php
@@ -0,0 +1,45 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Common\Util;
+
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Common\Util\StringUtilsTrait;
+use function strlen;
+
+class StringUtilsTraitTest extends TestCase
+{
+    use StringUtilsTrait;
+
+    /**
+     * @test
+     * @dataProvider provideLengths
+     */
+    public function generateRandomStringGeneratesStringOfProvidedLength(int $length)
+    {
+        $this->assertEquals($length, strlen($this->generateRandomString($length)));
+    }
+
+    public function provideLengths(): array
+    {
+        return [
+            [1],
+            [10],
+            [15],
+        ];
+    }
+
+    /**
+     * @test
+     */
+    public function generatesUuidV4()
+    {
+        $uuidPattern = '/[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{4}-[0-9A-Fa-f]{12}/';
+
+        $this->assertRegExp($uuidPattern, $this->generateV4Uuid());
+        $this->assertRegExp($uuidPattern, $this->generateV4Uuid());
+        $this->assertRegExp($uuidPattern, $this->generateV4Uuid());
+        $this->assertRegExp($uuidPattern, $this->generateV4Uuid());
+        $this->assertRegExp($uuidPattern, $this->generateV4Uuid());
+    }
+}
diff --git a/module/Core/src/Entity/Visit.php b/module/Core/src/Entity/Visit.php
index 3fd98db0..d7c44ff7 100644
--- a/module/Core/src/Entity/Visit.php
+++ b/module/Core/src/Entity/Visit.php
@@ -118,4 +118,12 @@ class Visit extends AbstractEntity implements JsonSerializable
             'remoteAddr' => null,
         ];
     }
+
+    /**
+     * @internal
+     */
+    public function getDate(): Chronos
+    {
+        return $this->date;
+    }
 }
diff --git a/module/Core/src/Exception/InvalidShortCodeException.php b/module/Core/src/Exception/InvalidShortCodeException.php
index a4e68cea..59a361bb 100644
--- a/module/Core/src/Exception/InvalidShortCodeException.php
+++ b/module/Core/src/Exception/InvalidShortCodeException.php
@@ -8,7 +8,7 @@ use function sprintf;
 
 class InvalidShortCodeException extends RuntimeException
 {
-    public static function fromCharset($shortCode, $charSet, Exception $previous = null)
+    public static function fromCharset(string $shortCode, string $charSet, Exception $previous = null): self
     {
         $code = $previous !== null ? $previous->getCode() : -1;
         return new static(
@@ -18,7 +18,7 @@ class InvalidShortCodeException extends RuntimeException
         );
     }
 
-    public static function fromNotFoundShortCode($shortCode)
+    public static function fromNotFoundShortCode(string $shortCode): self
     {
         return new static(sprintf('Provided short code "%s" does not belong to a short URL', $shortCode));
     }
diff --git a/module/Core/src/Exception/InvalidUrlException.php b/module/Core/src/Exception/InvalidUrlException.php
index 65f87e4e..6a94b548 100644
--- a/module/Core/src/Exception/InvalidUrlException.php
+++ b/module/Core/src/Exception/InvalidUrlException.php
@@ -10,7 +10,7 @@ class InvalidUrlException extends RuntimeException
 {
     public static function fromUrl($url, Throwable $previous = null)
     {
-        $code = isset($previous) ? $previous->getCode() : -1;
+        $code = $previous !== null ? $previous->getCode() : -1;
         return new static(sprintf('Provided URL "%s" is not an existing and valid URL', $url), $code, $previous);
     }
 }
diff --git a/module/Core/test/Entity/VisitTest.php b/module/Core/test/Entity/VisitTest.php
new file mode 100644
index 00000000..c5daaad6
--- /dev/null
+++ b/module/Core/test/Entity/VisitTest.php
@@ -0,0 +1,40 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Core\Entity;
+
+use Cake\Chronos\Chronos;
+use Shlinkio\Shlink\Core\Entity\ShortUrl;
+use Shlinkio\Shlink\Core\Entity\Visit;
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Core\Model\Visitor;
+
+class VisitTest extends TestCase
+{
+    /**
+     * @test
+     * @dataProvider provideDates
+     */
+    public function isProperlyJsonSerialized(?Chronos $date)
+    {
+        $visit = new Visit(new ShortUrl(''), new Visitor('Chrome', 'some site', '1.2.3.4'), $date);
+
+        $this->assertEquals([
+            'referer' => 'some site',
+            'date' => ($date ?? $visit->getDate())->toAtomString(),
+            'userAgent' => 'Chrome',
+            'visitLocation' => null,
+
+            // Deprecated
+            'remoteAddr' => null,
+        ], $visit->jsonSerialize());
+    }
+
+    public function provideDates(): array
+    {
+        return [
+            [null],
+            [Chronos::now()->subDays(10)],
+        ];
+    }
+}
diff --git a/module/Core/test/Exception/DeleteShortUrlExceptionTest.php b/module/Core/test/Exception/DeleteShortUrlExceptionTest.php
index 80c5ff10..f5c1ef80 100644
--- a/module/Core/test/Exception/DeleteShortUrlExceptionTest.php
+++ b/module/Core/test/Exception/DeleteShortUrlExceptionTest.php
@@ -21,6 +21,7 @@ class DeleteShortUrlExceptionTest extends TestCase
     ) {
         $e = DeleteShortUrlException::fromVisitsThreshold($threshold, $shortCode);
         $this->assertEquals($expectedMessage, $e->getMessage());
+        $this->assertEquals(0, $e->getCode());
     }
 
     public function provideMessages(): array
diff --git a/module/Core/test/Exception/InvalidShortCodeExceptionTest.php b/module/Core/test/Exception/InvalidShortCodeExceptionTest.php
new file mode 100644
index 00000000..4dca8e07
--- /dev/null
+++ b/module/Core/test/Exception/InvalidShortCodeExceptionTest.php
@@ -0,0 +1,43 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Core\Exception;
+
+use Exception;
+use Shlinkio\Shlink\Core\Exception\InvalidShortCodeException;
+use PHPUnit\Framework\TestCase;
+use Throwable;
+
+class InvalidShortCodeExceptionTest extends TestCase
+{
+    /**
+     * @test
+     * @dataProvider providePrevious
+     */
+    public function properlyCreatesExceptionFromCharset(?Throwable $prev)
+    {
+        $e = InvalidShortCodeException::fromCharset('abc123', 'def456', $prev);
+
+        $this->assertEquals('Provided short code "abc123" does not match the char set "def456"', $e->getMessage());
+        $this->assertEquals($prev !== null ? $prev->getCode() : -1, $e->getCode());
+        $this->assertEquals($prev, $e->getPrevious());
+    }
+
+    public function providePrevious(): array
+    {
+        return [
+            [null],
+            [new Exception('Previos error', 10)],
+        ];
+    }
+
+    /**
+     * @test
+     */
+    public function properlyCreatesExceptionFromNotFoundShortCode()
+    {
+        $e = InvalidShortCodeException::fromNotFoundShortCode('abc123');
+
+        $this->assertEquals('Provided short code "abc123" does not belong to a short URL', $e->getMessage());
+    }
+}
diff --git a/module/Core/test/Exception/InvalidUrlExceptionTest.php b/module/Core/test/Exception/InvalidUrlExceptionTest.php
new file mode 100644
index 00000000..67a12601
--- /dev/null
+++ b/module/Core/test/Exception/InvalidUrlExceptionTest.php
@@ -0,0 +1,33 @@
+<?php
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Core\Exception;
+
+use Exception;
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Core\Exception\InvalidUrlException;
+use Throwable;
+
+class InvalidUrlExceptionTest extends TestCase
+{
+    /**
+     * @test
+     * @dataProvider providePrevious
+     */
+    public function properlyCreatesExceptionFromUrl(?Throwable $prev)
+    {
+        $e = InvalidUrlException::fromUrl('http://the_url.com', $prev);
+
+        $this->assertEquals('Provided URL "http://the_url.com" is not an existing and valid URL', $e->getMessage());
+        $this->assertEquals($prev !== null ? $prev->getCode() : -1, $e->getCode());
+        $this->assertEquals($prev, $e->getPrevious());
+    }
+
+    public function providePrevious(): array
+    {
+        return [
+            [null],
+            [new Exception('Previos error', 10)],
+        ];
+    }
+}