diff --git a/composer.json b/composer.json
index 2db9ced..b567ef7 100644
--- a/composer.json
+++ b/composer.json
@@ -14,7 +14,7 @@
         "silverstripe/cms": "^4.0"
     },
     "require-dev": {
-        "sminnee/phpunit": "^5.7",
+        "phpunit/phpunit": "^9",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "autoload": {
diff --git a/tests/IFramePageTest.php b/tests/IFramePageTest.php
index 1d0c461..3eaff01 100644
--- a/tests/IFramePageTest.php
+++ b/tests/IFramePageTest.php
@@ -19,12 +19,12 @@ public function testGetClass()
         $iframe->AutoHeight = 1;
         $iframe->getClass();
 
-        $this->assertContains('iframepage-height-auto', $iframe->getClass());
+        $this->assertStringContainsString('iframepage-height-auto', $iframe->getClass());
 
         $iframe->AutoHeight = 0;
         $iframe->getClass();
 
-        $this->assertNotContains('iframepage-height-auto', $iframe->getClass());
+        $this->assertStringNotContainsString('iframepage-height-auto', $iframe->getClass());
     }
 
     public function testGetStyle()
@@ -33,19 +33,19 @@ public function testGetStyle()
 
         $iframe->FixedHeight = 0;
         $iframe->getStyle();
-        $this->assertContains('height: 800px', $iframe->getStyle(), 'Height defaults to 800 if not set.');
+        $this->assertStringContainsString('height: 800px', $iframe->getStyle(), 'Height defaults to 800 if not set.');
 
         $iframe->FixedHeight = 100;
         $iframe->getStyle();
-        $this->assertContains('height: 100px', $iframe->getStyle(), 'Fixed height is settable');
+        $this->assertStringContainsString('height: 100px', $iframe->getStyle(), 'Fixed height is settable');
 
         $iframe->AutoWidth = 1;
         $iframe->FixedWidth = '200';
-        $this->assertContains('width: 100%', $iframe->getStyle(), 'Auto width overrides fixed width');
+        $this->assertStringContainsString('width: 100%', $iframe->getStyle(), 'Auto width overrides fixed width');
 
         $iframe->AutoWidth = 0;
         $iframe->FixedWidth = '200';
-        $this->assertContains('width: 200px', $iframe->getStyle(), 'Fixed width is settable');
+        $this->assertStringContainsString('width: 200px', $iframe->getStyle(), 'Fixed width is settable');
     }
 
     public function testAllowedUrls()
@@ -85,7 +85,7 @@ public function testAllowedUrls()
 
         foreach ($tests['banned'] as $url) {
             $iframe->IFrameURL = $url;
-            $this->setExpectedException(ValidationException::class);
+            $this->expectException(ValidationException::class);
             $iframe->write();
         }
     }