diff --git a/apps/files_external/lib/Lib/Storage/FTP.php b/apps/files_external/lib/Lib/Storage/FTP.php
index 2761f0123b97a..66e623e5a1b2c 100644
--- a/apps/files_external/lib/Lib/Storage/FTP.php
+++ b/apps/files_external/lib/Lib/Storage/FTP.php
@@ -49,7 +49,8 @@ public function __construct(array $parameters) {
 				$this->secure = false;
 			}
 			$this->root = isset($parameters['root']) ? '/' . ltrim($parameters['root']) : '/';
-			$this->port = $parameters['port'] ?? 21;
+			$parsedPort = $parameters['port'] ?? null;
+			$this->port = is_numeric($parsedPort) ? (int)$parsedPort : 21;
 			$this->utf8Mode = isset($parameters['utf8']) && $parameters['utf8'];
 		} else {
 			throw new \Exception('Creating ' . self::class . ' storage failed, required parameters not set');
diff --git a/apps/files_external/tests/FtpConstructorTest.php b/apps/files_external/tests/FtpConstructorTest.php
new file mode 100644
index 0000000000000..78c9d6b33e030
--- /dev/null
+++ b/apps/files_external/tests/FtpConstructorTest.php
@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Files_External\Tests;
+
+use OCA\Files_External\Lib\Storage\FTP;
+use PHPUnit\Framework\Attributes\DataProvider;
+use Test\TestCase;
+
+/**
+ * The FTP connection is established lazily, so the constructor can be covered
+ * without a reachable FTP server. The backend tests in Storage/FtpTest.php do
+ * need one and are run separately.
+ */
+class FtpConstructorTest extends TestCase {
+	public static function portProvider(): array {
+		$parameters = [
+			'host' => 'somehost',
+			'user' => 'someuser',
+			'password' => 'somepassword',
+		];
+
+		return [
+			'no port given' => [$parameters, 21],
+			'empty port' => [array_merge($parameters, ['port' => '']), 21],
+			'null port' => [array_merge($parameters, ['port' => null]), 21],
+			'non numeric port' => [array_merge($parameters, ['port' => 'ftp']), 21],
+			'numeric string port' => [array_merge($parameters, ['port' => '2121']), 2121],
+			'integer port' => [array_merge($parameters, ['port' => 2121]), 2121],
+		];
+	}
+
+	#[DataProvider('portProvider')]
+	public function testPort(array $parameters, int $expectedPort): void {
+		$instance = new FTP($parameters);
+
+		$this->assertSame($expectedPort, self::invokePrivate($instance, 'port'));
+	}
+}