diff --git a/core/cmdbsource.class.inc.php b/core/cmdbsource.class.inc.php
index b82c665a0..cfb0cbd9e 100644
--- a/core/cmdbsource.class.inc.php
+++ b/core/cmdbsource.class.inc.php
@@ -154,6 +154,16 @@ class CMDBSource
 
 	/** @var mysqli $m_oMysqli */
 	protected static $m_oMysqli;
+	/**
+	 * The mysqli object is really hard to mock ! This attribute is used only in certain methods, so that we can mock only a very little subset of the mysqli object.
+	 * We are setting it in {@see Init}, by default it is a copy of {@see $m_oMysqli}
+	 * The mock can be injected using the setter {@see SetMySQLiForQuery}
+	 *
+	 * @var mysqli $oMySQLiForQuery
+	 * @see GetMySQLiForQuery
+	 * @see SetMySQLiForQuery
+	 * @since 2.7.5 N°3513 new var to allow mock in tests ({@see \Combodo\iTop\Test\UnitTest\Core\TransactionsTest})
+	 */
 	protected static $oMySQLiForQuery;
 
 	/**
@@ -235,6 +245,8 @@ class CMDBSource
 	 *
 	 * @return \mysqli
 	 * @throws \MySQLException
+	 *
+	 * @uses IsOpenedDbConnectionUsingTls when asking for a TLS connection, to check if it was really opened using TLS
 	 */
 	public static function GetMysqliInstance(
 		$sDbHost, $sUser, $sPwd, $sSource = '', $bTlsEnabled = false, $sTlsCa = null, $bCheckTlsAfterConnection = false
@@ -337,41 +349,41 @@ class CMDBSource
 	 * parameters were used.<br>
 	 * This method can be called to ensure that the DB connection really uses TLS.
 	 *
-	 * <p>We're using this object connection : {@link self::$m_oMysqli}
+	 * <p>We're using our own mysqli instance to do the check as this check is done when creating the mysqli instance : the consumer
+	 * might want a dedicated object, and if so we don't want to overwrite the one saved in CMDBSource !<br>
+	 * This is the case for example with {@see \iTopMutex} !
 	 *
 	 * @param \mysqli $oMysqli
 	 *
-	 * @return boolean true if the connection was really established using TLS
+	 * @return boolean true if the connection was really established using TLS, false otherwise
 	 * @throws \MySQLException
 	 *
+	 * @used-by GetMysqliInstance
 	 * @uses IsMySqlVarNonEmpty
+	 * @uses 'ssl_version' MySQL var
+	 * @uses 'ssl_cipher' MySQL var
 	 */
 	private static function IsOpenedDbConnectionUsingTls($oMysqli)
 	{
-		if (self::$m_oMysqli == null)
-		{
-			self::$m_oMysqli = $oMysqli;
-		}
-
-		$bNonEmptySslVersionVar = self::IsMySqlVarNonEmpty('ssl_version');
-		$bNonEmptySslCipherVar = self::IsMySqlVarNonEmpty('ssl_cipher');
+		$bNonEmptySslVersionVar = self::IsMySqlVarNonEmpty('ssl_version', $oMysqli);
+		$bNonEmptySslCipherVar = self::IsMySqlVarNonEmpty('ssl_cipher', $oMysqli);
 
 		return ($bNonEmptySslVersionVar && $bNonEmptySslCipherVar);
 	}
 
 	/**
 	 * @param string $sVarName
+	 * @param mysqli $oMysqli connection to use for the query
 	 *
 	 * @return bool
 	 * @throws \MySQLException
-	 *
-	 * @uses SHOW STATUS queries
+	 * @uses 'SHOW SESSION STATUS' queries
 	 */
-	private static function IsMySqlVarNonEmpty($sVarName)
+	private static function IsMySqlVarNonEmpty($sVarName, $oMysqli)
 	{
 		try
 		{
-			$sResult = self::QueryToScalar("SHOW SESSION STATUS LIKE '$sVarName'", 1);
+			$sResult = self::QueryToScalar("SHOW SESSION STATUS LIKE '$sVarName'", 1, $oMysqli);
 		}
 		catch (MySQLQueryHasNoResultException $e)
 		{
@@ -709,7 +721,7 @@ class CMDBSource
 		{
 			$aContext = array('query' => $sSql);
 
-			$iMySqlErrorNo = self::$m_oMysqli->errno;
+			$iMySqlErrorNo = self::GetMySQLiForQuery()->errno;
 			$aMySqlHasGoneAwayErrorCodes = MySQLHasGoneAwayException::getErrorCodes();
 			if (in_array($iMySqlErrorNo, $aMySqlHasGoneAwayErrorCodes))
 			{
@@ -731,7 +743,7 @@ class CMDBSource
 	private static function LogDeadLock(Exception $e)
 	{
 		// checks MySQL error code
-		$iMySqlErrorNo = self::$m_oMysqli->errno;
+		$iMySqlErrorNo = self::GetMySQLiForQuery()->errno;
 		if (!in_array($iMySqlErrorNo, array(self::MYSQL_ERRNO_WAIT_TIMEOUT, self::MYSQL_ERRNO_DEADLOCK)))
 		{
 			return;
@@ -739,7 +751,7 @@ class CMDBSource
 
 		// Get error info
 		$sUser = UserRights::GetUser();
-		$oError = self::$m_oMysqli->query('SHOW ENGINE INNODB STATUS');
+		$oError = self::GetMySQLiForQuery()->query('SHOW ENGINE INNODB STATUS');
 		if ($oError !== false)
 		{
 			$aData = $oError->fetch_all(MYSQLI_ASSOC);
@@ -967,17 +979,21 @@ class CMDBSource
 	/**
 	 * @param string $sSql
 	 * @param int $iCol beginning at 0
+	 * @param mysqli $oMysqli if not null will query using this connection, otherwise will use {@see GetMySQLiForQuery}
 	 *
 	 * @return string corresponding cell content on the first line
 	 * @throws \MySQLException
 	 * @throws \MySQLQueryHasNoResultException
+	 * @since 2.7.5-2 2.7.6 3.0.0 N°4215 new optional mysqli param
 	 */
-	public static function QueryToScalar($sSql, $iCol = 0)
+	public static function QueryToScalar($sSql, $iCol = 0, $oMysqli = null)
 	{
+		$oMysqliToQuery = (is_null($oMysqli)) ? self::GetMySQLiForQuery() : $oMysqli;
+
 		$oKPI = new ExecutionKPI();
 		try
 		{
-			$oResult = self::GetMySQLiForQuery()->query($sSql);
+			$oResult = $oMysqliToQuery->query($sSql);
 		}
 		catch(mysqli_sql_exception $e)
 		{
diff --git a/test/core/CMDBSource/CMDBSourceTest.php b/test/core/CMDBSource/CMDBSourceTest.php
index a787c0316..592786b02 100644
--- a/test/core/CMDBSource/CMDBSourceTest.php
+++ b/test/core/CMDBSource/CMDBSourceTest.php
@@ -5,6 +5,7 @@ namespace Combodo\iTop\Test\UnitTest\Core;
 
 use CMDBSource;
 use Combodo\iTop\Test\UnitTest\ItopTestCase;
+use utils;
 
 /**
  * @since 2.7.0
@@ -115,4 +116,23 @@ class CMDBSourceTest extends ItopTestCase
 			),
 		);
 	}
+
+	/**
+	 * @throws \ConfigException
+	 * @throws \CoreException
+	 * @throws \MySQLException
+	 * @since 3.0.0 N°4215
+	 */
+	public function testIsOpenedDbConnectionUsingTls() {
+		$oConfig = utils::GetConfig();
+		CMDBSource::InitFromConfig($oConfig);
+		$oMysqli = CMDBSource::GetMysqli();
+
+		// resets \CMDBSource::$oMySQLiForQuery to simulate call to \CMDBSource::Init with a TLS connexion
+		$this->InvokeNonPublicStaticMethod(CMDBSource::class, 'SetMySQLiForQuery',[null]);
+
+		// before N°4215 fix, this was crashing : "Call to a member function query() on null"
+		$bIsTlsCnx = $this->InvokeNonPublicStaticMethod(CMDBSource::class, 'IsOpenedDbConnectionUsingTls',[$oMysqli]);
+		$this->assertFalse($bIsTlsCnx);
+	}
 }