diff --git a/Sources/Avatar.php b/Sources/Avatar.php
index 7df9e0605e..940361a969 100644
--- a/Sources/Avatar.php
+++ b/Sources/Avatar.php
@@ -587,9 +587,14 @@ public function __construct(
 					break;
 
 				// Last ditch fallback is a transparent 1x1 GIF.
+				//
+				// This leaves the loop rather than the switch, because there is
+				// nothing left to try and because a data URI is not a URL that
+				// Url::isValid() will accept, so the condition the loop tests
+				// can never become false from here.
 				default:
 					$url = new Url('data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7');
-					break;
+					break 2;
 			}
 		}
 
diff --git a/tests/Unit/AvatarFallbackTest.php b/tests/Unit/AvatarFallbackTest.php
new file mode 100644
index 0000000000..d66465eefe
--- /dev/null
+++ b/tests/Unit/AvatarFallbackTest.php
@@ -0,0 +1,110 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SMF\Tests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use SMF\Avatar;
+use SMF\Config;
+
+/**
+ * Covers what SMF\Avatar settles on when it can find no image at all.
+ *
+ * The constructor only asks the database who owns an attachment, so handing it
+ * an id_member keeps the whole thing on this side of the line.
+ */
+#[CoversClass(Avatar::class)]
+class AvatarFallbackTest extends TestCase
+{
+	/*********************
+	 * Internal properties
+	 *********************/
+
+	/**
+	 * @var array The settings this class reads, as they were before the test.
+	 */
+	private array $backup = [];
+
+	/**
+	 * @var string Config::$boardurl as it was before the test.
+	 */
+	private string $boardurl = '';
+
+	/****************
+	 * Public methods
+	 ****************/
+
+	/**
+	 * The constructor tries each way of finding an image in turn, and stops
+	 * when it has a URL that validates. The last of those ways is a 1x1
+	 * transparent GIF as a data URI, which is there so that there is always an
+	 * answer - but a data URI is not something Url::isValid() accepts, so it
+	 * never satisfied the condition that ends the search and the search went
+	 * round again, and again, until the request was killed.
+	 *
+	 * Reaching it takes nothing exotic: the step before only produces a URL
+	 * when avatar_url is set, so a forum where that setting was never written
+	 * hung on any member with an avatar.
+	 */
+	public function testAnAvatarThatCannotBeFoundEndsAsATransparentGif(): void
+	{
+		Config::$boardurl = 'https://example.com';
+		Config::$modSettings['gravatarEnabled'] = false;
+		unset(Config::$modSettings['avatar_url']);
+
+		$avatar = new Avatar(url: 'Oxygen/beagle.png', id_member: 1);
+
+		$this->assertStringStartsWith('data:image/gif;base64,', (string) $avatar->url);
+	}
+
+	/**
+	 * The control, and the reason the loop exists: when there is somewhere to
+	 * look, it is looked in, and the search ends long before the last resort.
+	 */
+	public function testAnAvatarThatCanBeFoundIsStillFound(): void
+	{
+		Config::$boardurl = 'https://example.com';
+		Config::$modSettings['gravatarEnabled'] = false;
+		Config::$modSettings['avatar_url'] = 'https://example.com/avatars';
+
+		$avatar = new Avatar(url: 'Oxygen/beagle.png', id_member: 1);
+
+		$this->assertSame('https://example.com/avatars/Oxygen/beagle.png', (string) $avatar->url);
+	}
+
+	/******************
+	 * Internal methods
+	 ******************/
+
+	protected function setUp(): void
+	{
+		$this->boardurl = Config::$boardurl ?? '';
+
+		foreach (['avatar_url', 'gravatarEnabled'] as $key) {
+			if (isset(Config::$modSettings[$key])) {
+				$this->backup[$key] = Config::$modSettings[$key];
+			}
+		}
+	}
+
+	/**
+	 * PHPUnit does not reset SMF's statics between tests, so a setting left
+	 * behind here would leak into every test that follows.
+	 */
+	protected function tearDown(): void
+	{
+		Config::$boardurl = $this->boardurl;
+
+		foreach (['avatar_url', 'gravatarEnabled'] as $key) {
+			unset(Config::$modSettings[$key]);
+
+			if (isset($this->backup[$key])) {
+				Config::$modSettings[$key] = $this->backup[$key];
+			}
+		}
+
+		$this->backup = [];
+	}
+}