diff --git a/includes/api/ApiLogout.php b/includes/api/ApiLogout.php
index 6c005bbb5b6..12397231135 100644
--- a/includes/api/ApiLogout.php
+++ b/includes/api/ApiLogout.php
@@ -51,6 +51,13 @@ class ApiLogout extends ApiBase {
 		}
 
 		$user = $this->getUser();
+
+		if ( $user->isAnon() ) {
+			// Cannot logout a anon user, so add a warning and return early.
+			$this->addWarning( 'apierror-mustbeloggedin-generic', 'notloggedin' );
+			return;
+		}
+
 		$oldName = $user->getName();
 		$user->logout();
 
diff --git a/tests/phpunit/includes/api/ApiLogoutTest.php b/tests/phpunit/includes/api/ApiLogoutTest.php
index 0fc3338456c..c56d9abda1e 100644
--- a/tests/phpunit/includes/api/ApiLogoutTest.php
+++ b/tests/phpunit/includes/api/ApiLogoutTest.php
@@ -35,6 +35,20 @@ class ApiLogoutTest extends ApiTestCase {
 		}
 	}
 
+	public function testUserLogoutAlreadyLoggedOut() {
+		$user = $this->getServiceContainer()->getUserFactory()->newAnonymous( '1.2.3.4' );
+
+		$this->assertFalse( $user->isRegistered() );
+		$token = $this->getUserCsrfTokenFromApi( $user );
+		$response = $this->doUserLogout( $token, $user )[0];
+		$this->assertFalse( $user->isRegistered() );
+
+		$this->assertArrayEquals(
+			[ 'warnings' => [ 'logout' => [ 'warnings' => 'You must be logged in.' ] ] ],
+			$response
+		);
+	}
+
 	public function testUserLogout() {
 		$user = $this->getTestSysop()->getUser();