diff --git a/apps/dav/lib/CalDAV/Federation/CalendarFederationProvider.php b/apps/dav/lib/CalDAV/Federation/CalendarFederationProvider.php
index aef3c75ca38..de5f5b987d2 100644
--- a/apps/dav/lib/CalDAV/Federation/CalendarFederationProvider.php
+++ b/apps/dav/lib/CalDAV/Federation/CalendarFederationProvider.php
@@ -63,6 +63,13 @@ class CalendarFederationProvider implements ICloudFederationProvider {
 		}
 
 		$rawProtocol = $share->getProtocol();
+		if (!isset($rawProtocol[ICalendarFederationProtocol::PROP_VERSION])) {
+			throw new ProviderCouldNotAddShareException(
+				'No protocol version',
+				'',
+				Http::STATUS_BAD_REQUEST,
+			);
+		}
 		switch ($rawProtocol[ICalendarFederationProtocol::PROP_VERSION]) {
 			case CalendarFederationProtocolV1::VERSION:
 				try {
diff --git a/apps/dav/lib/CalDAV/Federation/FederationSharingService.php b/apps/dav/lib/CalDAV/Federation/FederationSharingService.php
index 8dcb8c7cebc..57df75b01a0 100644
--- a/apps/dav/lib/CalDAV/Federation/FederationSharingService.php
+++ b/apps/dav/lib/CalDAV/Federation/FederationSharingService.php
@@ -46,10 +46,10 @@ class FederationSharingService {
 	 */
 	private function decodeRemoteUserPrincipal(string $principal): ?string {
 		// Expected format: principals/remote-users/abcdef123
-		[$prefix, $collection, $encodedId] = explode('/', $principal);
-		if ($prefix !== 'principals' || $collection !== 'remote-users') {
+		if (!str_starts_with($principal, 'principals/remote-users/')) {
 			return null;
 		}
+		$encodedId = substr($principal, strlen('principals/remote-users/'));
 
 		$decodedId = base64_decode($encodedId);
 		if (!is_string($decodedId)) {
diff --git a/apps/dav/tests/unit/CalDAV/Federation/CalendarFederationProviderTest.php b/apps/dav/tests/unit/CalDAV/Federation/CalendarFederationProviderTest.php
index aba2f927934..8e436897789 100644
--- a/apps/dav/tests/unit/CalDAV/Federation/CalendarFederationProviderTest.php
+++ b/apps/dav/tests/unit/CalDAV/Federation/CalendarFederationProviderTest.php
@@ -175,7 +175,7 @@ class CalendarFederationProviderTest extends TestCase {
 			->method('add');
 
 		$this->expectException(ProviderCouldNotAddShareException::class);
-		$this->expectExceptionMessage('Unknown protocol version');
+		$this->expectExceptionMessage('No protocol version');
 		$this->expectExceptionCode(400);
 		$this->assertEquals(10, $this->calendarFederationProvider->shareReceived($share));
 	}