diff --git a/api/src/CalDAV/JsBase.php b/api/src/CalDAV/JsBase.php
index e9f5357488..a504a4a1e3 100644
--- a/api/src/CalDAV/JsBase.php
+++ b/api/src/CalDAV/JsBase.php
@@ -200,7 +200,7 @@ class JsBase
 
 		foreach($definitions as $name => $definition)
 		{
-			$data = $cfs[$name];
+			$data = $cfs[$name] ?? null;
 			if (isset($data))
 			{
 				if (is_scalar($data) || is_array($data) && !isset($data['value']))
@@ -358,7 +358,15 @@ class JsBase
 			}
 			if (isset($value))
 			{
-				$target = $value;
+				// objects need to be merged, to not unset all not given attributes
+				if (is_array($value) && !array_key_exists(0, $value))
+				{
+					$target = array_merge($target, $value);
+				}
+				else
+				{
+					$target = $value;
+				}
 			}
 			else
 			{
diff --git a/api/src/CalDAV/JsCalendar.php b/api/src/CalDAV/JsCalendar.php
index d9353d39d2..81ccefeeff 100644
--- a/api/src/CalDAV/JsCalendar.php
+++ b/api/src/CalDAV/JsCalendar.php
@@ -179,7 +179,7 @@ class JsCalendar extends JsBase
 						break;
 
 					case 'egroupware.org:customfields':
-						$event += self::parseCustomfields($value, $strict);
+						$event = array_merge($event, self::parseCustomfields($value, $strict));
 						break;
 
 					case 'prodId':
diff --git a/api/src/Contacts/JsContact.php b/api/src/Contacts/JsContact.php
index 6c79233a62..530271a68f 100644
--- a/api/src/Contacts/JsContact.php
+++ b/api/src/Contacts/JsContact.php
@@ -188,7 +188,7 @@ class JsContact extends Api\CalDAV\JsBase
 						break;
 
 					case 'egroupware.org:customfields':
-						$contact += self::parseCustomfields($value);
+						$contact = array_merge($contact, self::parseCustomfields($value));
 						break;
 
 					case 'egroupware.org:assistant':
diff --git a/timesheet/src/JsTimesheet.php b/timesheet/src/JsTimesheet.php
index 702f357536..9454b2a669 100644
--- a/timesheet/src/JsTimesheet.php
+++ b/timesheet/src/JsTimesheet.php
@@ -154,7 +154,7 @@ class JsTimesheet extends Api\CalDAV\JsBase
 						break;
 
 					case 'egroupware.org:customfields':
-						$timesheet += self::parseCustomfields($value);
+						$timesheet = array_merge($timesheet, self::parseCustomfields($value));
 						break;
 
 					case 'prodId':
@@ -163,6 +163,7 @@ class JsTimesheet extends Api\CalDAV\JsBase
 					case 'modifier':
 					case self::AT_TYPE:
 					case 'id':
+					case 'etag':
 						break;
 
 					default: