diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 05e7b40..3584e6c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -31,4 +31,4 @@ jobs:
       os: >-
         ['ubuntu-latest', 'windows-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/.github/workflows/composer-require-checker.yml b/.github/workflows/composer-require-checker.yml
index a857bce..a93390b 100644
--- a/.github/workflows/composer-require-checker.yml
+++ b/.github/workflows/composer-require-checker.yml
@@ -31,4 +31,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/.github/workflows/rector.yml b/.github/workflows/rector.yml
index 457772a..5d6931d 100644
--- a/.github/workflows/rector.yml
+++ b/.github/workflows/rector.yml
@@ -21,4 +21,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.3']
+        ['8.4']
diff --git a/.github/workflows/static.yml b/.github/workflows/static.yml
index e33eca8..d03874d 100644
--- a/.github/workflows/static.yml
+++ b/.github/workflows/static.yml
@@ -29,4 +29,4 @@ jobs:
       os: >-
         ['ubuntu-latest']
       php: >-
-        ['8.1', '8.2', '8.3']
+        ['8.1', '8.2', '8.3', '8.4']
diff --git a/CHANGELOG.md b/CHANGELOG.md
index e12ea1c..2b23d70 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,8 @@
 - Enh #62: Use `SensitiveParameter` attribute to mark sensitive parameters (@dehbka, @vjik)
 - Chg #66: Bump minimal required PHP version to 8.1 (@vjik)
 - Enh #66: Mark readonly properties (@vjik)
+- Chg #67: Change PHP constraint in `composer.json` to `8.1 - 8.4` (@vjik)
+- Bug #67: Explicitly mark nullable parameters (@vjik)
  
 ## 1.0.2 March 18, 2024
 
diff --git a/composer.json b/composer.json
index 62423e0..28eeea5 100644
--- a/composer.json
+++ b/composer.json
@@ -34,7 +34,7 @@
         }
     ],
     "require": {
-        "php": "^8.1",
+        "php": "8.1 - 8.4",
         "ext-hash": "*",
         "ext-openssl": "*",
         "yiisoft/strings": "^2.0"
@@ -45,7 +45,7 @@
         "rector/rector": "^2.0.9",
         "roave/infection-static-analysis-plugin": "^1.35",
         "spatie/phpunit-watcher": "^1.24",
-        "vimeo/psalm": "^5.26.1"
+        "vimeo/psalm": "^5.26.1 || ^6.8.8"
     },
     "autoload": {
         "psr-4": {
diff --git a/psalm.xml b/psalm.xml
index b48c894..679cd9b 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -3,6 +3,7 @@
     errorLevel="1"
     findUnusedBaselineEntry="true"
     findUnusedCode="false"
+    strictBinaryOperands="false"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="https://getpsalm.org/schema/config"
     xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
diff --git a/src/Crypt.php b/src/Crypt.php
index 423a57e..803730b 100644
--- a/src/Crypt.php
+++ b/src/Crypt.php
@@ -240,6 +240,10 @@ private function encrypt(
 
         $encrypted = openssl_encrypt($data, $this->cipher, $key, OPENSSL_RAW_DATA, $iv);
         if ($encrypted === false) {
+            /**
+             * @psalm-suppress PossiblyFalseOperand `openssl_encrypt()` is returned `false`, so `openssl_error_string()`
+             * always returns string.
+             */
             throw new \RuntimeException('OpenSSL failure on encryption: ' . openssl_error_string());
         }
 
@@ -300,6 +304,10 @@ private function decrypt(
 
         $decrypted = openssl_decrypt($encrypted, $this->cipher, $key, OPENSSL_RAW_DATA, $iv);
         if ($decrypted === false) {
+            /**
+             * @psalm-suppress PossiblyFalseOperand `openssl_decrypt()` is returned `false`, so `openssl_error_string()`
+             * always returns string.
+             */
             throw new \RuntimeException('OpenSSL failure on decryption: ' . openssl_error_string());
         }
 
diff --git a/src/PasswordHasher.php b/src/PasswordHasher.php
index ac44ef2..a024a6a 100644
--- a/src/PasswordHasher.php
+++ b/src/PasswordHasher.php
@@ -28,7 +28,7 @@ final class PasswordHasher
      */
     public function __construct(
         private readonly ?string $algorithm = PASSWORD_DEFAULT,
-        array $parameters = null,
+        ?array $parameters = null,
     ) {
         if ($parameters === null) {
             $this->parameters = self::SAFE_PARAMETERS[$this->algorithm] ?? [];