diff --git a/composer.json b/composer.json
index 0c2a374..53d408e 100644
--- a/composer.json
+++ b/composer.json
@@ -21,6 +21,7 @@
     ],
     "require": {
         "php": "^7.1 || ^8.0",
+        "ext-json": "*",
         "paragonie/random-lib": "^2.0",
         "symfony/polyfill-mbstring": "^1.19"
     },
diff --git a/src/Factory.php b/src/Factory.php
index 5686323..352924a 100644
--- a/src/Factory.php
+++ b/src/Factory.php
@@ -27,17 +27,21 @@ final class Factory
      */
     private static $factory;
 
+    /**
+     * @throws InvalidShortidException
+     */
     public function generate(int $length = null, string $alphabet = null, bool $readable = null): Shortid
     {
-        $length = null === $length ? $this->length : $length;
-        $readable = null === $readable ? $this->readable : $readable;
+        $length = $length ?? $this->length;
+        $readable = $readable ?? $this->readable;
         if (null === $alphabet && $readable) {
-            $alphabet = Generator::EASY_TO_READ;
+            $alphabet = \str_replace(\str_split(Generator::AMBIGUOUS_CHARS), '', $this->alphabet);
+            $alphabet .= \str_repeat('_', \strlen(Generator::AMBIGUOUS_CHARS) / 2);
+            $alphabet .= \str_repeat('-', \strlen(Generator::AMBIGUOUS_CHARS) / 2);
         }
-        $alphabet = null === $alphabet ? $this->alphabet : $alphabet;
-        $id = self::getFactory()->getMediumStrengthGenerator()->generateString($length, $alphabet);
+        $id = self::getFactory()->getMediumStrengthGenerator()->generateString($length, $alphabet ?? $this->alphabet);
 
-        return new Shortid($id);
+        return new Shortid($id, $length, $alphabet);
     }
 
     public function setAlphabet(string $alphabet): void
diff --git a/src/InvalidShortidException.php b/src/InvalidShortidException.php
new file mode 100644
index 0000000..6f4edb9
--- /dev/null
+++ b/src/InvalidShortidException.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace PUGX\Shortid;
+
+final class InvalidShortidException extends \Exception
+{
+}
diff --git a/src/Shortid.php b/src/Shortid.php
index 78fb472..10d1415 100644
--- a/src/Shortid.php
+++ b/src/Shortid.php
@@ -14,8 +14,15 @@ final class Shortid implements \JsonSerializable, \Serializable
      */
     private static $factory;
 
-    public function __construct(string $id)
+    /**
+     * @throws InvalidShortidException
+     */
+    public function __construct(string $id, int $length = null, string $alphabet = null)
     {
+        if (!self::isValid($id, $length, $alphabet)) {
+            throw new InvalidShortidException(\sprintf('Invalid shortid %s (length %d alphabet %s', $id, $length, $alphabet));
+        }
+
         $this->id = $id;
     }
 
@@ -24,6 +31,9 @@ public function __toString(): string
         return $this->id;
     }
 
+    /**
+     * @throws InvalidShortidException
+     */
     public static function generate(int $length = null, string $alphabet = null, bool $readable = false): self
     {
         if (null === $length) {
diff --git a/tests/FactoryTest.php b/tests/FactoryTest.php
index 65a57d6..2ea75a7 100644
--- a/tests/FactoryTest.php
+++ b/tests/FactoryTest.php
@@ -22,7 +22,7 @@ public function testGenerate(): void
     {
         $generated = $this->factory->generate();
 
-        $this->assertRegExp('/^[a-z0-9\_\-]{7,7}$/i', $generated->__toString());
+        self::assertRegExp('/^[a-z0-9\_\-]{7,7}$/i', $generated->__toString());
     }
 
     /**
@@ -57,7 +57,7 @@ public function testSetAlphabet(string $alphabet): void
 
         $newAlphabet = $this->factory->getAlphabet();
 
-        $this->assertSame($alphabet, $newAlphabet);
+        self::assertSame($alphabet, $newAlphabet);
     }
 
     /**
@@ -77,7 +77,7 @@ public function wrongAlphabetsProvider(): array
     {
         return [
             'test' => ['test'],
-            'rand' => [\sha1((string) \random_int(0, \getrandmax()))],
+            'rand' => [\sha1((string) \random_int(0, \mt_getrandmax()))],
         ];
     }
 
@@ -85,19 +85,19 @@ public function testGetFactory(): void
     {
         $factory = Factory::getFactory();
 
-        $this->assertInstanceOf(RandomLibFactory::class, $factory);
+        self::assertInstanceOf(RandomLibFactory::class, $factory);
     }
 
     public function testSetLength(): void
     {
         $this->factory->setLength(5);
-        $this->assertSame(5, $this->factory->getLength());
+        self::assertSame(5, $this->factory->getLength());
     }
 
     public function testCheckLength(): void
     {
         $null = $this->factory->checkLength(null, false);
-        $this->assertNull($null);
+        self::assertNull($null);
     }
 
     public function testSetWrongLengthType(): void
diff --git a/tests/ShortidTest.php b/tests/ShortidTest.php
index 9dc4eeb..b4fd690 100644
--- a/tests/ShortidTest.php
+++ b/tests/ShortidTest.php
@@ -5,6 +5,7 @@
 use JsonSerializable;
 use PHPUnit\Framework\TestCase;
 use PUGX\Shortid\Factory;
+use PUGX\Shortid\InvalidShortidException;
 use PUGX\Shortid\Shortid;
 
 final class ShortidTest extends TestCase
@@ -18,28 +19,28 @@ public function testGenerate(): void
     {
         $generated = Shortid::generate();
 
-        $this->assertRegExp('/^[a-z0-9\_\-]{7}$/i', $generated->__toString());
+        self::assertRegExp('/^[a-z0-9\_\-]{7}$/i', $generated->__toString());
     }
 
     public function testGenerateWithReadable(): void
     {
         $generated = Shortid::generate(null, null, true);
 
-        $this->assertRegExp('/^[a-z0-9\_\-]{7}$/i', $generated->__toString());
+        self::assertRegExp('/^[a-z0-9\_\-]{7}$/i', $generated->__toString());
     }
 
     public function testGenerateWithLength(): void
     {
         $generated = Shortid::generate(8);
 
-        $this->assertRegExp('/^[a-z0-9\_\-]{8}$/i', $generated->__toString());
+        self::assertRegExp('/^[a-z0-9\_\-]{8}$/i', $generated->__toString());
     }
 
     public function testGetFactory(): void
     {
         $factory = Shortid::getFactory();
 
-        $this->assertInstanceOf(Factory::class, $factory);
+        self::assertInstanceOf(Factory::class, $factory);
     }
 
     public function testSetFactory(): void
@@ -47,18 +48,18 @@ public function testSetFactory(): void
         $factory = new Factory();
         Shortid::setFactory($factory);
 
-        $this->assertSame($factory, Shortid::getFactory());
+        self::assertSame($factory, Shortid::getFactory());
     }
 
     public function testIsValid(): void
     {
-        $this->assertTrue(Shortid::isValid('shortid'));
+        self::assertTrue(Shortid::isValid('shortid'));
     }
 
     public function testIsNotValid(): void
     {
-        $this->assertFalse(Shortid::isValid('/(;#!'));
-        $this->assertFalse(Shortid::isValid('harmful string stuff'));
+        self::assertFalse(Shortid::isValid('/(;#!'));
+        self::assertFalse(Shortid::isValid('harmful string stuff'));
     }
 
     public function testIsValidWithRegexChar(): void
@@ -67,28 +68,28 @@ public function testIsValidWithRegexChar(): void
         $factory->setAlphabet('hìjklmnòpqrstùvwxyzABCDEFGHIJKLMNOPQRSTUVWX.\+*?[^]$(){}=!<>|:-/');
         Shortid::setFactory($factory);
 
-        $this->assertTrue(Shortid::isValid('slsh/]?'));
+        self::assertTrue(Shortid::isValid('slsh/]?'));
     }
 
     public function testJsonSerializable(): void
     {
         $generated = Shortid::generate();
 
-        $this->assertInstanceOf(JsonSerializable::class, $generated);
+        self::assertInstanceOf(JsonSerializable::class, $generated);
     }
 
     public function testJsonEncode(): void
     {
         $generated = Shortid::generate();
 
-        $this->assertSame('"'.$generated.'"', \json_encode($generated));
+        self::assertSame('"'.$generated.'"', \json_encode($generated));
     }
 
     public function testSerialize(): void
     {
         $shortid = new Shortid('shortid');
 
-        $this->assertSame('shortid', $shortid->serialize());
+        self::assertSame('shortid', $shortid->serialize());
     }
 
     public function testUnserialize(): void
@@ -96,14 +97,14 @@ public function testUnserialize(): void
         $shortid = Shortid::generate();
         $shortid->unserialize('shortid');
 
-        $this->assertSame('shortid', (string) $shortid);
+        self::assertSame('shortid', (string) $shortid);
     }
 
     public function testMagicSerialize(): void
     {
         $shortid = new Shortid('shortid');
 
-        $this->assertSame(['id' => 'shortid'], $shortid->__serialize());
+        self::assertSame(['id' => 'shortid'], $shortid->__serialize());
     }
 
     public function testMagicUnserialize(): void
@@ -111,6 +112,12 @@ public function testMagicUnserialize(): void
         $shortid = Shortid::generate();
         $shortid->__unserialize(['id' => 'shortid']);
 
-        $this->assertSame('shortid', (string) $shortid);
+        self::assertSame('shortid', (string) $shortid);
+    }
+
+    public function testInvalidArgumentInConstructor(): void
+    {
+        $this->expectException(InvalidShortidException::class);
+        new Shortid('an_invalid_too_long_shortid');
     }
 }