diff --git a/src/ProcessMaker/Nayra/Storage/BpmnDocument.php b/src/ProcessMaker/Nayra/Storage/BpmnDocument.php
index b3e2f55..0425d35 100644
--- a/src/ProcessMaker/Nayra/Storage/BpmnDocument.php
+++ b/src/ProcessMaker/Nayra/Storage/BpmnDocument.php
@@ -392,8 +392,8 @@ class BpmnDocument extends DOMDocument implements BpmnDocumentInterface
                     DataInputAssociationInterface::BPMN_PROPERTY_TRANSFORMATION => ['1', [self::BPMN_MODEL, DataInputAssociationInterface::BPMN_PROPERTY_TRANSFORMATION]],
                 ],
             ],
-            DataInputAssociationInterface::BPMN_PROPERTY_TARGET_REF => [self::IS_REFERENCE, []],
-            DataInputAssociationInterface::BPMN_PROPERTY_SOURCES_REF => [self::IS_REFERENCE, []],
+            DataInputAssociationInterface::BPMN_PROPERTY_TARGET_REF => [self::IS_REFERENCE_OPTIONAL, []],
+            DataInputAssociationInterface::BPMN_PROPERTY_SOURCES_REF => [self::IS_REFERENCE_OPTIONAL, []],
             DataInputAssociationInterface::BPMN_PROPERTY_TRANSFORMATION => [
                 FormalExpressionInterface::class,
                 [
@@ -546,6 +546,8 @@ class BpmnDocument extends DOMDocument implements BpmnDocumentInterface
 
     const IS_REFERENCE = 'isReference';
 
+    const IS_REFERENCE_OPTIONAL = 'isReferenceOptional';
+
     const TEXT_PROPERTY = 'textProperty';
 
     const IS_ARRAY = 'isArray';
@@ -703,7 +705,7 @@ public function hasBpmnInstance($id)
      *
      * @return \ProcessMaker\Nayra\Contracts\Bpmn\EntityInterface
      */
-    public function getElementInstanceById($id)
+    public function getElementInstanceById($id, ?bool $isOptional = false)
     {
         $this->bpmnElements[$id] = isset($this->bpmnElements[$id])
             ? $this->bpmnElements[$id]
@@ -712,7 +714,7 @@ public function getElementInstanceById($id)
                 ? $element->getBpmnElementInstance()
                 : null
             );
-        if ($this->bpmnElements[$id] === null && empty($element)) {
+        if ($this->bpmnElements[$id] === null && empty($element) && !$isOptional) {
             throw new ElementNotFoundException($id);
         }
 
diff --git a/src/ProcessMaker/Nayra/Storage/BpmnElement.php b/src/ProcessMaker/Nayra/Storage/BpmnElement.php
index c7b71e1..121736d 100644
--- a/src/ProcessMaker/Nayra/Storage/BpmnElement.php
+++ b/src/ProcessMaker/Nayra/Storage/BpmnElement.php
@@ -44,9 +44,11 @@ public function getBpmnElementInstance($owner = null)
             return null;
         }
         list($classInterface, $mapProperties) = $map[$this->namespaceURI][$this->localName];
-        if ($classInterface === BpmnDocument::IS_REFERENCE) {
-            $bpmnElement = $this->ownerDocument->getElementInstanceById($this->nodeValue);
-            $this->bpmn = $bpmnElement;
+        if ($classInterface === BpmnDocument::IS_REFERENCE || $classInterface === BpmnDocument::IS_REFERENCE_OPTIONAL) {
+            $bpmnElement = $this->ownerDocument->getElementInstanceById(
+                $this->nodeValue,
+                $classInterface === BpmnDocument::IS_REFERENCE_OPTIONAL
+            );
         } elseif ($classInterface === BpmnDocument::TEXT_PROPERTY) {
             $bpmnElement = $this->nodeValue;
             $owner->setProperty($this->nodeName, $this->nodeValue);