diff --git a/src/Core/Extensible.php b/src/Core/Extensible.php
index 95782feee30..98163ba7067 100644
--- a/src/Core/Extensible.php
+++ b/src/Core/Extensible.php
@@ -511,7 +511,7 @@ public function getExtensionInstances()
         // Setup all extension instances for this instance
         $this->extension_instances = [];
         foreach (ClassInfo::ancestry(static::class) as $class) {
-            if (in_array($class, self::$unextendable_classes)) {
+            if (in_array($class, self::class::$unextendable_classes)) {
                 continue;
             }
             $extensions = Config::inst()->get($class, 'extensions', Config::UNINHERITED | Config::EXCLUDE_EXTRA_SOURCES);
diff --git a/src/Dev/Deprecation.php b/src/Dev/Deprecation.php
index e8d1111a431..05050c4f193 100644
--- a/src/Dev/Deprecation.php
+++ b/src/Dev/Deprecation.php
@@ -391,7 +391,7 @@ public static function notice($atVersion, $string = '', $scope = Deprecation::SC
                 }
 
                 // Getting a backtrace is slow, so we only do it if we need it
-                $backtrace = null;
+                $backtrace = [];
 
                 // Get the calling scope
                 if ($scope == Deprecation::SCOPE_METHOD) {
diff --git a/tests/php/Dev/DeprecationTest.php b/tests/php/Dev/DeprecationTest.php
index 03f0154fd48..811e2ca70b6 100644
--- a/tests/php/Dev/DeprecationTest.php
+++ b/tests/php/Dev/DeprecationTest.php
@@ -111,8 +111,8 @@ public function testNoticeNoReplacement()
             'Will be removed without equivalent functionality to replace it.',
             'Called from SilverStripe\Dev\Tests\DeprecationTest->testNoticeNoReplacement.'
         ]);
-        $this->expectDeprecation();
-        $this->expectDeprecationMessage($message);
+        $this->expectException(DeprecationTestException::class);
+        $this->expectExceptionMessage($message);
         $this->enableDeprecationNotices(true);
         $ret = $this->myDeprecatedMethodNoReplacement();
         $this->assertSame('abc', $ret);