diff --git a/src/ai/api/ai_tile.hpp b/src/ai/api/ai_tile.hpp
index 11bdf58def..c979549f2d 100644
--- a/src/ai/api/ai_tile.hpp
+++ b/src/ai/api/ai_tile.hpp
@@ -80,7 +80,7 @@ public:
 		SLOPE_STEEP_E  = SLOPE_STEEP | SLOPE_SEN,               //!< A steep slope falling to west (from east)
 		SLOPE_STEEP_N  = SLOPE_STEEP | SLOPE_ENW,               //!< A steep slope falling to south (from north)
 
-		SLOPE_INVALID  = 0xFF,                                  //!< An invalid slope
+		SLOPE_INVALID  = 0xFFFF,                                //!< An invalid slope
 	};
 
 	/**