diff --git a/core/oqlclasstreeoptimizer.class.inc.php b/core/oqlclasstreeoptimizer.class.inc.php
index 1e12384e9..4bc070218 100644
--- a/core/oqlclasstreeoptimizer.class.inc.php
+++ b/core/oqlclasstreeoptimizer.class.inc.php
@@ -55,7 +55,7 @@ class OQLClassTreeOptimizer
 						$sJoinedClass = $oJoin->GetOOQLClassNode()->GetNodeClass();
 						$sExtKeyAttCode = $oJoin->GetLeftField();
 						$oExtKeyAttDef = MetaModel::GetAttributeDef($oCurrentClassNode->GetNodeClass(), $sExtKeyAttCode);
-						if ($sJoinedClass == $oExtKeyAttDef->GetTargetClass()) {
+						if (($oExtKeyAttDef instanceof AttributeExternalKey) && ($sJoinedClass == $oExtKeyAttDef->GetTargetClass())) {
 							// The join is not used, remove from tree
 							$oCurrentClassNode->RemoveJoin($sLeftKey, $index);
 						}
diff --git a/test/core/DBSearchTest.php b/test/core/DBSearchTest.php
index abc8a7795..91bfd370f 100644
--- a/test/core/DBSearchTest.php
+++ b/test/core/DBSearchTest.php
@@ -720,4 +720,19 @@ class DBSearchTest extends ItopDataTestCase
 		/** @var \DBObjectSearch $oNestedSearchInExpression */
 		self::assertEquals($bAllowAllData, $oNestedSearchInExpression->IsAllDataAllowed(), 'Nested DBSearch AllowData value');
 	}
+
+	/**
+	 * BUG N°4031 check AttributeObjectKey used in JOIN condition
+	 * @throws \ConfigException
+	 * @throws \CoreException
+	 * @throws \MissingQueryArgument
+	 * @throws \OQLException
+	 */
+	public function testAttributeObjectKey()
+	{
+		$sQuery = "SELECT II FROM InlineImage AS II JOIN UserRequest AS UR ON II.item_id = UR.id WHERE II.item_class = 'UserRequest'";
+		$oSearch = \DBObjectSearch::FromOQL($sQuery);
+		$oSearch->MakeSelectQuery();
+		self::assertTrue(true);
+	}
 }