diff --git a/src/Domain/User/UserRepository.php b/src/Domain/User/UserRepository.php
index 5e8ec43f..54e7dc93 100644
--- a/src/Domain/User/UserRepository.php
+++ b/src/Domain/User/UserRepository.php
@@ -425,12 +425,18 @@ class UserRepository {
 	/**
 	 * Fetch a line by id.
 	 *
-	 * @param int $rID Line id.
+	 * @param int|string|null $rID Line id. Callers hand over nullable columns (a
+	 *                             device's `pair_id`) and request values; anything
+	 *                             that is not a positive id finds nothing.
 	 * @return array|null The line row, or null if not found.
 	 */
-	public static function getLineById(int $rID) {
+	public static function getLineById($rID) {
+		if (!is_numeric($rID) || (int) $rID <= 0) {
+			return null;
+		}
+
 		$db = self::db();
-		$db->query('SELECT * FROM `lines` WHERE `id` = ?;', $rID);
+		$db->query('SELECT * FROM `lines` WHERE `id` = ?;', (int) $rID);
 
 		if ($db->num_rows() == 1) {
 			return $db->get_row();
diff --git a/tests/Unit/UserRepositoryTest.php b/tests/Unit/UserRepositoryTest.php
index 8a262f8c..8f731264 100644
--- a/tests/Unit/UserRepositoryTest.php
+++ b/tests/Unit/UserRepositoryTest.php
@@ -111,4 +111,16 @@ final class UserRepositoryTest extends TestCase {
 		// An id-based lookup carries no username/password to re-verify.
 		$this->assertTrue($this->call('verifyCachedCredentials', array(), 5, null, null));
 	}
+
+	/**
+	 * getLineById() is handed nullable columns (a MAG / Enigma2 device's
+	 * `pair_id`, an activation code's `subscriber_id`) and raw request values.
+	 * None of those is a line, and none may end the request: loading an
+	 * unpaired MAG device used to throw, so deleting one answered an empty page.
+	 */
+	public function testGetLineByIdFindsNothingForANonId(): void {
+		foreach (array(null, '', 0, '0', -3, 'abc') as $rID) {
+			$this->assertNull(UserRepository::getLineById($rID), var_export($rID, true));
+		}
+	}
 }