diff --git a/apps/user_ldap/lib/Access.php b/apps/user_ldap/lib/Access.php
index 09fd099b7506a..04546937b1029 100644
--- a/apps/user_ldap/lib/Access.php
+++ b/apps/user_ldap/lib/Access.php
@@ -703,7 +703,7 @@ private function ldap2NextcloudNames(array $ldapObjects, bool $isUsers): array {
 					if (is_null($nameByLDAP)) {
 						continue;
 					}
-					$sndName = $ldapObject[$sndAttribute][0] ?? '';
+					$sndName = $sndAttribute !== null ? ($ldapObject[$sndAttribute][0] ?? '') : '';
 					$this->applyUserDisplayName($ncName, $nameByLDAP, $sndName);
 				} elseif ($nameByLDAP !== null) {
 					$this->cacheGroupDisplayName($ncName, $nameByLDAP);
diff --git a/lib/private/DB/QueryBuilder/QueryBuilder.php b/lib/private/DB/QueryBuilder/QueryBuilder.php
index 30e85675a75dc..7719703e2c2a0 100644
--- a/lib/private/DB/QueryBuilder/QueryBuilder.php
+++ b/lib/private/DB/QueryBuilder/QueryBuilder.php
@@ -725,7 +725,7 @@ public function join($fromAlias, $join, $alias, $condition = null) {
 		$this->queryBuilder->join(
 			$this->quoteAlias($fromAlias),
 			$this->getTableName($join),
-			$this->quoteAlias($alias),
+			$this->quoteAlias($alias) ?? '',
 			$condition
 		);
 
@@ -754,7 +754,7 @@ public function innerJoin($fromAlias, $join, $alias, $condition = null) {
 		$this->queryBuilder->innerJoin(
 			$this->quoteAlias($fromAlias),
 			$this->getTableName($join),
-			$this->quoteAlias($alias),
+			$this->quoteAlias($alias) ?? '',
 			$condition
 		);
 
@@ -783,7 +783,7 @@ public function leftJoin($fromAlias, $join, $alias, $condition = null) {
 		$this->queryBuilder->leftJoin(
 			$this->quoteAlias($fromAlias),
 			$this->getTableName($join),
-			$this->quoteAlias($alias),
+			$this->quoteAlias($alias) ?? '',
 			$condition
 		);
 
@@ -812,7 +812,7 @@ public function rightJoin($fromAlias, $join, $alias, $condition = null) {
 		$this->queryBuilder->rightJoin(
 			$this->quoteAlias($fromAlias),
 			$this->getTableName($join),
-			$this->quoteAlias($alias),
+			$this->quoteAlias($alias) ?? '',
 			$condition
 		);
 
diff --git a/lib/public/Cache/CappedMemoryCache.php b/lib/public/Cache/CappedMemoryCache.php
index cbffece2c0fa9..49e06c6f90b65 100644
--- a/lib/public/Cache/CappedMemoryCache.php
+++ b/lib/public/Cache/CappedMemoryCache.php
@@ -37,6 +37,9 @@ public function __construct(int $capacity = 512) {
 	 */
 	#[\Override]
 	public function hasKey($key): bool {
+		if ($key === null) {
+			return false;
+		}
 		return isset($this->cache[$key]);
 	}
 
@@ -46,6 +49,9 @@ public function hasKey($key): bool {
 	 */
 	#[\Override]
 	public function get($key) {
+		if ($key === null) {
+			return null;
+		}
 		return $this->cache[$key] ?? null;
 	}
 
@@ -73,6 +79,9 @@ public function set($key, $value, $ttl = 0): bool {
 	 */
 	#[\Override]
 	public function remove($key): bool {
+		if ($key === null) {
+			return false;
+		}
 		unset($this->cache[$key]);
 		return true;
 	}
diff --git a/tests/lib/DB/QueryBuilder/QueryBuilderTest.php b/tests/lib/DB/QueryBuilder/QueryBuilderTest.php
index 0014ce86420dc..4c57a652ba4b6 100644
--- a/tests/lib/DB/QueryBuilder/QueryBuilderTest.php
+++ b/tests/lib/DB/QueryBuilder/QueryBuilderTest.php
@@ -532,7 +532,7 @@ public static function dataJoin(): array {
 		return [
 			[
 				'd1', 'data2', null, null,
-				['`d1`' => [['joinType' => 'inner', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => null, 'joinCondition' => null]]],
+				['`d1`' => [['joinType' => 'inner', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => '', 'joinCondition' => null]]],
 				'`*PREFIX*data1` `d1` INNER JOIN `*PREFIX*data2` '
 			],
 			[
@@ -613,7 +613,7 @@ public static function dataLeftJoin(): array {
 		return [
 			[
 				'd1', 'data2', null, null,
-				['`d1`' => [['joinType' => 'left', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => null, 'joinCondition' => null]]],
+				['`d1`' => [['joinType' => 'left', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => '', 'joinCondition' => null]]],
 				'`*PREFIX*data1` `d1` LEFT JOIN `*PREFIX*data2` '
 			],
 			[
@@ -663,7 +663,7 @@ public static function dataRightJoin(): array {
 		return [
 			[
 				'd1', 'data2', null, null,
-				['`d1`' => [['joinType' => 'right', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => null, 'joinCondition' => null]]],
+				['`d1`' => [['joinType' => 'right', 'joinTable' => '`*PREFIX*data2`', 'joinAlias' => '', 'joinCondition' => null]]],
 				'`*PREFIX*data1` `d1` RIGHT JOIN `*PREFIX*data2` '
 			],
 			[