diff --git a/spritesheet.lua b/spritesheet.lua
index 37e8948..53b4ef4 100644
--- a/spritesheet.lua
+++ b/spritesheet.lua
@@ -297,7 +297,7 @@ function SpriteSheet:setOrigin (ox, oy, mode)
         self.origin_x = self.tile_width * ox
         self.origin_y = self.tile_height * oy
     else
-        error('Unknown origin mode "%s"', mode)
+        error('Unknown origin mode %s (%s)', mode, type(mode))
     end
 end