diff --git a/.github/workflows/psalm.yml b/.github/workflows/psalm.yml
new file mode 100644
index 0000000..051dfdc
--- /dev/null
+++ b/.github/workflows/psalm.yml
@@ -0,0 +1,33 @@
+name: Psalm
+
+on:
+  push:
+    paths:
+      - '**.php'
+      - 'psalm.xml.dist'
+
+jobs:
+  psalm:
+    name: psalm
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          extensions: json
+          coverage: none
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: vendor
+          key: composer-${{ hashFiles('composer.lock') }}
+
+      - name: Run composer install
+        run: composer install -n --prefer-dist
+
+      - name: Run psalm
+        run: ./vendor/bin/psalm --output-format=github
diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
new file mode 100644
index 0000000..c3278d5
--- /dev/null
+++ b/.github/workflows/run-tests.yml
@@ -0,0 +1,38 @@
+name: run-tests
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 0 * * *"
+
+jobs:
+  test:
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository
+    timeout-minutes: 15
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ["7.4", "8.0", "8.1"]
+        dependency-version: ["prefer-lowest", "prefer-stable"]
+        os: ["ubuntu-latest", "windows-latest"]
+
+    name: P${{ matrix.php }} - ${{ matrix.dependency-version }} - ${{ matrix.os }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: json
+          coverage: none
+
+      - name: Install dependencies
+        run: composer update --${{ matrix.dependency-version }} --prefer-dist --no-interaction --no-suggest
+
+      - name: Execute tests
+        run: vendor/bin/phpunit
diff --git a/.gitignore b/.gitignore
index d769eb5..dddbc6a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,3 @@
 vendor
-.phpunit.result.cache
+.phpunit.cache
 composer.lock
diff --git a/composer.json b/composer.json
index 11b3173..7670e2e 100644
--- a/composer.json
+++ b/composer.json
@@ -4,11 +4,12 @@
     "type": "library",
     "keywords": [],
     "require": {
-        "php": ">=7.4",
+        "php": "^7.4 || ^8.0",
         "ext-json": "*",
         "psr/http-factory": "^1.0",
         "psr/http-client": "^1.0",
-        "guzzlehttp/guzzle": "^6.0 || ^7.0"
+        "guzzlehttp/guzzle": "^6.5.2 || ^7.0",
+        "guzzlehttp/psr7": "^1.7.0"
     },
     "require-dev": {
         "http-interop/http-factory-guzzle": "^1.0",
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
new file mode 100644
index 0000000..8d3e365
--- /dev/null
+++ b/phpunit.xml.dist
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.5/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         cacheResultFile=".phpunit.cache/test-results"
+         convertDeprecationsToExceptions="true"
+         failOnRisky="true"
+         failOnWarning="true"
+         verbose="true"
+         colors="true"
+>
+    <testsuites>
+        <testsuite name="default">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <coverage cacheDirectory=".phpunit.cache/code-coverage"
+              processUncoveredFiles="true">
+        <include>
+            <directory suffix=".php">src</directory>
+        </include>
+    </coverage>
+</phpunit>
diff --git a/psalm.xml b/psalm.xml.dist
similarity index 100%
rename from psalm.xml
rename to psalm.xml.dist
diff --git a/src/Offers/OffersListDto.php b/src/Offers/OffersListDto.php
index 6271462..0879229 100644
--- a/src/Offers/OffersListDto.php
+++ b/src/Offers/OffersListDto.php
@@ -36,7 +36,7 @@ class OffersListDto extends BaseOfferDto
     protected int $noticePercentOvercap;
     protected float $cr;
     protected float $epc;
-    protected ?string $notes;
+    protected string $notes;
     protected string $allowedIp;
     protected string $disallowedIp;
     protected string $hashPassword;
@@ -86,7 +86,7 @@ public function __construct(array $attributes)
         $this->noticePercentOvercap = $attributes['notice_percent_overcap'];
         $this->cr = $attributes['cr'];
         $this->epc = $attributes['epc'];
-        $this->notes = $attributes['notes'];
+        $this->notes = $attributes['notes'] ?? '';
         $this->allowedIp = $attributes['allowed_ip'];
         $this->disallowedIp = $attributes['disallowed_ip'];
         $this->hashPassword = $attributes['hash_password'];