diff --git a/lib/Controllers/DefaultController.php b/lib/Controllers/DefaultController.php
index cda1ea9..870f648 100644
--- a/lib/Controllers/DefaultController.php
+++ b/lib/Controllers/DefaultController.php
@@ -807,19 +807,18 @@ class DefaultController extends ControllerAbstract {
 
     private function nodeList(string $tenantId, string $userId, array $data, int $version, string $transaction): StreamedNdJsonResponse {
 
-        if (isset($data['sources'])) {
-            if (!is_array($data['sources'])) {
-                throw new InvalidArgumentException(self::ERR_INVALID_SOURCES);
-            }
+        if (!isset($data['sources'])) {
+            throw new InvalidArgumentException(self::ERR_MISSING_SOURCES);
+        }
+        if (!is_array($data['sources'])) {
+            throw new InvalidArgumentException(self::ERR_INVALID_SOURCES);
+        }
 
-            $sources = ResourceIdentifiers::fromArray($data['sources']);
-            foreach ($sources as $source) {
-                if (!$source instanceof ServiceIdentifier && !$source instanceof CollectionIdentifier) {
-                    throw new InvalidArgumentException('Invalid parameter: sources must contain provider:service or provider:service:collection identifiers');
-                }
+        $sources = ResourceIdentifiers::fromArray($data['sources']);
+        foreach ($sources as $source) {
+            if ($source instanceof EntityIdentifier || (!$source instanceof ServiceIdentifier && !$source instanceof CollectionIdentifier)) {
+                throw new InvalidArgumentException('Invalid parameter: sources must contain provider:service or provider:service:collection identifiers');
             }
-        } else {
-            $sources = null;
         }
 
         $filter = $data['filter'] ?? null;