Calendar export: `Argument #3 ($subject) must be of type array|string, null given`

Steps to reproduce

1. click on the calendar edit button (pencil) on the left
2. click on Export

or

1. just try to download the calendar via the URL using auth

![NOTE] This error does not happen for all calendars

Expected behavior

Nextcloud should export the calendar and the download should start

Actual behavior

HTTP error 500 (see log below)

Calendar app version

4.7.16

CalDAV-clients used

Web UI

Browser

No response

Client operating system

Debian

Server operating system

Debian

Web server

Apache

Database engine version

PostgreSQL

PHP engine version

None

Nextcloud version

29.0.14

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

Nextcloud configuration

Web server error log

Log file

{"reqId":"x","level":3,"time":"x","remoteAddr":"x","user":"x",
"app":"webdav","method":"GET","url":"/remote.php/dav/calendars/x/n/?export",
"message":"str_replace(): Argument #3 ($subject) must be of type array|string, null given",
"userAgent":"Mozilla/5.0 (X11; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0",
"version":"29.0.14.1",
"exception":{"Exception":"TypeError","Message":"str_replace(): Argument #cal/}calendar-color":"#9B9B9B",
"0":"And 1 more entries, set log level to debug to see all entries"},
{"__class__":"Sabre3 ($subject) must be of type array|string, null given","Code":0,
"Trace":[{"file":"/var/www/html/3rdparty/sabre/dav/lib/CalDAV/Plugin.php","line":420,
"function":"str_replace","args":["\r","",null]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/PropFind.php","line":95,
"function":"Sabre\\CalDAV\\{closure}","class":"Sabre\\CalDAV\\Plugin",
"type":"->","args":["*** sensitive parameters replaced ***"]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/CalDAV/Plugin.php",
"line":405,"function":"handle","class":"Sabre\\DAV\\PropFind",
"type":"->","args":["{urn:ietf:params:xml:ns:caldav}calendar-data",
{"__class__":"Closure"}]},
{"file":"/var/www/html/3rdparty/sabre/event/lib/WildcardEmitterTrait.php","line":89,
"function":"propFind","class":"Sabre\\CalDAV\\Plugin","type":"->",
"args":[{"__class__":"Sabre\\DAV\\PropFind"},
{"__class__":"OCA\\DAV\\CalDAV\\CalendarObject"}]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php","line":1052,
"function":"emit","class":"Sabre\\DAV\\Server","type":"->",
"args":["propFind",[{"__class__":"Sabre\\DAV\\PropFind"},
{"__class__":"OCA\\DAV\\CalDAV\\CalendarObject"}]]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php",
"line":984,"function":"getPropertiesByNode",
"class":"Sabre\\DAV\\Server","type":"->",
"args":[{"__class__":"Sabre\\DAV\\PropFind"},
{"__class__":"OCA\\DAV\\CalDAV\\CalendarObject"}]},
{"function":"getPropertiesIteratorForPath","class":"Sabre\\DAV\\Server","type":"->",
"args":["calendars/x/n",["{urn:ietf:params:xml:ns:caldav}calendar-data"],1]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php","line":941,
"function":"iterator_to_array","args":[{"__class__":"Generator"}]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/CalDAV/ICSExportPlugin.php",
"line":214,"function":"getPropertiesForPath","class":"Sabre\\DAV\\Server",
"type":"->","args":["calendars/x/n",["{urn:ietf:params:xml:ns:caldav}calendar-data"],1]},
{"file":"/var/www/html/apps/dav/lib/CalDAV/ICSExportPlugin/ICSExportPlugin.php","line":61,
"function":"generateResponse","class":"Sabre\\CalDAV\\ICSExportPlugin","type":"->",
"args":["calendars/x/n",null,null,false,false,"text/calendar",
{"{DAV:}resourcetype":{"__class__":"Sabre\\DAV\\Xml\\Property\\ResourceType"},
"{DAV:}displayname":"N","{http://sabredav.org/ns}sync-token":399,
"{DAV:}sync-token":"http://sabre.io/ns/sync/399",
"{http://apple.com/ns/ical/}calendar-color":"#9B9B9B","0":"And 1 more entries, set log level to debug to see all entries"},
{"__class__":"Sabre\\HTTP\\Response"}]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/CalDAV/ICSExportPlugin.php",
"line":155,"function":"generateResponse","class":"OCA\\DAV\\CalDAV\\ICSExportPlugin\\ICSExportPlugin",
"type":"->","args":["calendars/x/n",null,null,false,false,"text/calendar",
{"{DAV:}resourcetype":{"__class__":"Sabre\\DAV\\Xml\\Property\\ResourceType"},
"{DAV:}displayname":"N",
"{http://sabredav.org/ns}sync-token":399,
"{DAV:}sync-token":"http://sabre.io/ns/sync/399",
"{http://apple.com/ns/i\\HTTP\\Response"}]},
{"file":"/var/www/html/3rdparty/sabre/event/lib/WildcardEmitterTrait.php",
"line":89,"function":"httpGet","class":"Sabre\\CalDAV\\ICSExportPlugin",
"type":"->","args":[{"__class__":"Sabre\\HTTP\\Request"},
{"__class__":"Sabre\\HTTP\\Response"}]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php",
"line":472,"function":"emit","class":"Sabre\\DAV\\Server",
"type":"->","args":["method:GET",[{"__class__":"Sabre\\HTTP\\Request"},
{"__class__":"Sabre\\HTTP\\Response"}]]},
{"file":"/var/www/html/apps/dav/lib/Connector/Sabre/Server.php",
"line":67,"function":"invokeMethod","class":"Sabre\\DAV\\Server",
"type":"->","args":[{"__class__":"Sabre\\HTTP\\Request"},
{"__class__":"Sabre\\HTTP\\Response"}]},
{"file":"/var/www/html/3rdparty/sabre/dav/lib/DAV/Server.php","line":321,
"function":"start","class":"OCA\\DAV\\Connector\\Sabre\\Server","type":"->","args":[]},
{"file":"/var/www/html/apps/dav/lib/Server.php","line":396,"function":"exec","class":"Sabre\\DAV\\Server",
"type":"->","args":[]},
"file":"/var/www/html/apps/dav/appinfo/v2/remote.php","line":35,"function":"exec","class":"OCA\\DAV\\Server",
"type":"->","args":[]},{"file":"/var/www/html/remote.php",
"line":172,"args":["/var/www/html/apps/dav/appinfo/v2/remote.php"],
"function":"require_once"}],
"File":"/var/www/html/3rdparty/sabre/dav/lib/CalDAV/Plugin.php",
"Line":420,
"message":"str_replace(): Argument #3 ($subject) must be of type array|string, null given",
"exception":{},
"CustomMessage":"str_replace(): Argument #3 ($subject) must be of type array|string, null given"}}

Browser log

Additional info

I'm using a Docker environment for Nextcloud, Postgres and Collabora.

I provided a redacted section of the log file containing the error.

This is a mitigation through which I can download the exported calendars, although I haven't checked if entries are missing or if the export is accurate:

@/var/www/3rdparty/sabre/dav/lib/CalDAV/Plugin.php LINE 411
- return str_replace("\r", '', $val);
+ if (is_null($val)) {
+     return $val;
+ } 
+ else {
+     return str_replace("\r", '', $val);
+ }

Before experiencing this error I migrated from PostgreSQL 13 on the host to PostgreSQL 17 on a Docker container. I did a database dump and imported it in the container. Apparently no errors were reported, however this might be a factor to consider.

I noticed this problem when one of my script reported a failure while exporting the calendars.

I'm not sure if the database migration caused the error, or if it's something else since my Systemd logs don't go far back.

Start by reproducing the authenticated export request at /remote.php/dav/calendars/.../?export and inspect 3rdparty/sabre/dav/lib/CalDAV/Plugin.php around line 420 and apps/dav/lib/CalDAV/ICSExportPlugin/ICSExportPlugin.php. Compare calendars that export successfully with the affected calendar and determine why calendar-data reaches str_replace() as null. Done means affected calendars export successfully without losing entries and the behavior is covered by an appropriate test.