diff --git a/src/Composer/Command/LicensesCommand.php b/src/Composer/Command/LicensesCommand.php
index 73adb4451..28f584c66 100644
--- a/src/Composer/Command/LicensesCommand.php
+++ b/src/Composer/Command/LicensesCommand.php
@@ -92,8 +92,7 @@ EOT
                 break;
 
             default:
-                $output->writeln(sprintf('Unsupported format "%s".  See help for supported formats.', $format));
-                break;
+                throw new \RuntimeException(sprintf('Unsupported format "%s".  See help for supported formats.', $format));
         }
     }
 }