diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..d981c87
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,31 @@
+
+name: CI
+on: [push, pull_request]
+
+permissions:
+  contents: read #  to fetch code (actions/checkout)
+
+jobs:
+    tests:
+        name: Tests
+        timeout-minutes: 10
+        runs-on: ${{ matrix.os }}
+        steps:
+            -   name: Checkout
+                uses: actions/checkout@v3
+            -   name: Setup PHP
+                uses: shivammathur/setup-php@v2
+                with:
+                    php-version: ${{ matrix.php-version }}
+            -   name: Composer Install
+                run: composer install --no-interaction --no-cache
+            -   name: Make Tests Compatiable With PHPUnit 9+
+                if: contains(fromJSON('["7.3", "7.4", "8.0", "8.1", "8.2", "8.3"]'), matrix.php-version)
+                run: php tests/make_compatible_with_phpunit9.php
+            -   name: PHPUnit
+                run: vendor/bin/phpunit
+        strategy:
+            fail-fast: false
+            matrix:
+                os: [ubuntu-latest, windows-latest, macos-latest]
+                php-version: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
\ No newline at end of file
diff --git a/README.md b/README.md
index 5289bda..a7d44af 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # mcrypt_compat
 
-[![Build Status](https://travis-ci.org/phpseclib/mcrypt_compat.svg?branch=master)](https://app.travis-ci.com/github/phpseclib/mcrypt_compat)
+[![CI Status](https://github.com/phpseclib/mcrypt_compat/actions/workflows/ci.yml/badge.svg?branch=1.0&event=push "CI Status")](https://github.com/phpseclib/mcrypt_compat/actions/workflows/ci.yml?query=branch%3A1.0)
 
 PHP 5.x-8.x polyfill for mcrypt extension.
 
diff --git a/lib/mcrypt.php b/lib/mcrypt.php
index e58796a..d014343 100644
--- a/lib/mcrypt.php
+++ b/lib/mcrypt.php
@@ -1178,7 +1178,7 @@ function phpseclib_mcrypt_ecb($cipher, $key, $data, $mode, $iv = null)
      */
     function phpseclib_mcrypt_encrypt($cipher, $key, $data, $mode, $iv = null)
     {
-        return defined('PHPSECLIB_MCRYPT_TARGET_VERSION') && version_compare(PHPSECLIB_MCRYPT_TARGET_VERSION, '5.6.0', '>=') ?
+        return !defined('PHPSECLIB_MCRYPT_TARGET_VERSION') || version_compare(PHPSECLIB_MCRYPT_TARGET_VERSION, '5.6.0', '>=') ?
                phpseclib_mcrypt_helper($cipher, $key, $data, $mode, $iv, 'encrypt') :
                phpseclib_mcrypt_helper_old($cipher, $key, $data, $mode, $iv, 'encrypt');
     }
@@ -1198,7 +1198,7 @@ function phpseclib_mcrypt_encrypt($cipher, $key, $data, $mode, $iv = null)
      */
     function phpseclib_mcrypt_decrypt($cipher, $key, $data, $mode, $iv = null)
     {
-        return defined('PHPSECLIB_MCRYPT_TARGET_VERSION') && version_compare(PHPSECLIB_MCRYPT_TARGET_VERSION, '5.6.0', '>=') ?
+        return !defined('PHPSECLIB_MCRYPT_TARGET_VERSION') || version_compare(PHPSECLIB_MCRYPT_TARGET_VERSION, '5.6.0', '>=') ?
                phpseclib_mcrypt_helper($cipher, $key, $data, $mode, $iv, 'decrypt') :
                phpseclib_mcrypt_helper_old($cipher, $key, $data, $mode, $iv, 'decrypt');
     }
diff --git a/tests/MCryptCompatTest.php b/tests/MCryptCompatTest.php
index d2a2b3d..17014ab 100644
--- a/tests/MCryptCompatTest.php
+++ b/tests/MCryptCompatTest.php
@@ -825,6 +825,28 @@ public function testMcryptGenericWithTwoParamsPHPPost71()
         phpseclib_mcrypt_generic_init($td, 'xxx');
     }
 
+    public function testOldMcryptNoIV()
+    {
+        $key = 'key';
+        $data = 'data';
+        $iv = null;
+
+        $this->setExpectedException('PHPUnit_Framework_Error_Warning', 'mcrypt_encrypt(): Attempt to use an empty IV, which is NOT recommended');
+
+        phpseclib_mcrypt_helper_old('rijndael-128', $key, $data, 'cbc', $iv, 'encrypt');
+    }
+
+    public function testOldMcryptShortIV()
+    {
+        $key = 'key';
+        $data = 'data';
+        $iv = 'iv';
+
+        $this->setExpectedException('PHPUnit_Framework_Error_Warning', 'mcrypt_encrypt(): The IV parameter must be as long as the blocksize');
+
+        phpseclib_mcrypt_helper_old('rijndael-128', $key, $data, 'cbc', $iv, 'encrypt');
+    }
+
     public function providerForIVSizeChecks()
     {
         $tests = [
diff --git a/tests/make_compatible_with_phpunit9.php b/tests/make_compatible_with_phpunit9.php
new file mode 100644
index 0000000..1d2fe26
--- /dev/null
+++ b/tests/make_compatible_with_phpunit9.php
@@ -0,0 +1,26 @@
+<?php
+
+/** @var iterable<SplFileInfo> $files */
+$files = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator(__DIR__));
+foreach ($files as $file) {
+    if ($file->getExtension() === 'php' && $file->getPathname() !== __FILE__) {
+        $fileContents = file_get_contents($file->getPathname());
+        if ($fileContents === false) {
+            throw new \RuntimeException('file_get_contents() failed: ' . $file->getPathname());
+        }
+        $patternToReplacementMap = array(
+            '~(n assertIsArray\([^,\)]*,)([^,\)]*\))~' => '$1 string $2: void',
+            '~(n assertIsArray\([^,\)]*\))~' => '$1: void',
+            '~(n assertIsString\([^\)]*\))~' => '$1: void',
+            '~(n assertStringContainsString\([^\)]*\))~' => '$1: void'
+        );
+        $updatedFileContents = preg_replace(
+            array_keys($patternToReplacementMap),
+            array_values($patternToReplacementMap),
+            $fileContents
+        );
+        if (file_put_contents($file->getPathname(), $updatedFileContents) === false) {
+            throw new \RuntimeException('file_put_contents() failed: ' . $file->getPathname());
+        }
+    }
+}
\ No newline at end of file