diff --git a/lib/private/Security/CredentialsManager.php b/lib/private/Security/CredentialsManager.php
index 0ac9b30c6c..df1d892656 100644
--- a/lib/private/Security/CredentialsManager.php
+++ b/lib/private/Security/CredentialsManager.php
@@ -53,7 +53,7 @@ class CredentialsManager implements ICredentialsManager {
 	/**
 	 * Store a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @param mixed $credentials
 	 */
@@ -61,7 +61,7 @@ class CredentialsManager implements ICredentialsManager {
 		$value = $this->crypto->encrypt(json_encode($credentials));
 
 		$this->dbConnection->setValues(self::DB_TABLE, [
-			'user' => $userId,
+			'user' => (string)$userId,
 			'identifier' => $identifier,
 		], [
 			'credentials' => $value,
@@ -71,7 +71,7 @@ class CredentialsManager implements ICredentialsManager {
 	/**
 	 * Retrieve a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @return mixed
 	 */
@@ -79,7 +79,7 @@ class CredentialsManager implements ICredentialsManager {
 		$qb = $this->dbConnection->getQueryBuilder();
 		$qb->select('credentials')
 			->from(self::DB_TABLE)
-			->where($qb->expr()->eq('user', $qb->createNamedParameter($userId)))
+			->where($qb->expr()->eq('user', $qb->createNamedParameter((string)$userId)))
 			->andWhere($qb->expr()->eq('identifier', $qb->createNamedParameter($identifier)))
 		;
 		$result = $qb->execute()->fetch();
@@ -95,14 +95,14 @@ class CredentialsManager implements ICredentialsManager {
 	/**
 	 * Delete a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @return int rows removed
 	 */
 	public function delete($userId, $identifier) {
 		$qb = $this->dbConnection->getQueryBuilder();
 		$qb->delete(self::DB_TABLE)
-			->where($qb->expr()->eq('user', $qb->createNamedParameter($userId)))
+			->where($qb->expr()->eq('user', $qb->createNamedParameter((string)$userId)))
 			->andWhere($qb->expr()->eq('identifier', $qb->createNamedParameter($identifier)))
 		;
 		return $qb->execute();
diff --git a/lib/public/Security/ICredentialsManager.php b/lib/public/Security/ICredentialsManager.php
index 6c1bb67192..17d1fa5a35 100644
--- a/lib/public/Security/ICredentialsManager.php
+++ b/lib/public/Security/ICredentialsManager.php
@@ -33,7 +33,7 @@ interface ICredentialsManager {
 	/**
 	 * Store a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @param mixed $credentials
 	 * @since 8.2.0
@@ -43,7 +43,7 @@ interface ICredentialsManager {
 	/**
 	 * Retrieve a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @return mixed
 	 * @since 8.2.0
@@ -53,7 +53,7 @@ interface ICredentialsManager {
 	/**
 	 * Delete a set of credentials
 	 *
-	 * @param string|null $userId Null for system-wide credentials
+	 * @param string $userId empty string for system-wide credentials
 	 * @param string $identifier
 	 * @return int rows removed
 	 * @since 8.2.0