diff --git a/src/Core/Config/Cache/ConfigCache.php b/src/Core/Config/Cache/ConfigCache.php
index bafb3fc957..3119b5db12 100644
--- a/src/Core/Config/Cache/ConfigCache.php
+++ b/src/Core/Config/Cache/ConfigCache.php
@@ -68,7 +68,7 @@ class ConfigCache
 	 *
 	 * @return null|mixed Returns the value of the Config entry or null if not set
 	 */
-	public function get(string $cat, $key = null)
+	public function get(string $cat, string $key = null)
 	{
 		if (isset($this->config[$cat][$key])) {
 			return $this->config[$cat][$key];
diff --git a/src/Core/Config/Cache/PConfigCache.php b/src/Core/Config/Cache/PConfigCache.php
index 4ee2bfae72..b03d86f41e 100644
--- a/src/Core/Config/Cache/PConfigCache.php
+++ b/src/Core/Config/Cache/PConfigCache.php
@@ -62,7 +62,7 @@ class PConfigCache
 	 *
 	 * @return null|string The value of the config entry or null if not set
 	 */
-	public function get(int $uid, string $cat, $key = null)
+	public function get(int $uid, string $cat, string $key = null)
 	{
 		if (isset($this->config[$uid][$cat][$key])) {
 			return $this->config[$uid][$cat][$key];
diff --git a/tests/src/Core/Config/Cache/ConfigCacheTest.php b/tests/src/Core/Config/Cache/ConfigCacheTest.php
index db92cc7436..d8ac3eaea4 100644
--- a/tests/src/Core/Config/Cache/ConfigCacheTest.php
+++ b/tests/src/Core/Config/Cache/ConfigCacheTest.php
@@ -157,6 +157,12 @@ class ConfigCacheTest extends MockedTest
 			'key1' => 'value1',
 			'key2' => 'value2',
 		], $configCache->get('system'));
+
+		// explicit null as key
+		$this->assertEquals([
+			'key1' => 'value1',
+			'key2' => 'value2',
+		], $configCache->get('system', null));
 	}
 
 	/**
diff --git a/tests/src/Core/Config/Cache/PConfigCacheTest.php b/tests/src/Core/Config/Cache/PConfigCacheTest.php
index e0e100bbac..22beccf601 100644
--- a/tests/src/Core/Config/Cache/PConfigCacheTest.php
+++ b/tests/src/Core/Config/Cache/PConfigCacheTest.php
@@ -79,6 +79,12 @@ class PConfigCacheTest extends MockedTest
 			'key1' => 'value1',
 			'key2' => 'value2',
 		], $configCache->get($uid, 'system'));
+
+		// test explicit cat with null as key
+		$this->assertEquals([
+			'key1' => 'value1',
+			'key2' => 'value2',
+		], $configCache->get($uid, 'system', null));
 	}
 
 	/**