diff --git a/src/newgrf_canal.cpp b/src/newgrf_canal.cpp
--- a/src/newgrf_canal.cpp
+++ b/src/newgrf_canal.cpp
@@ -48,7 +48,7 @@ static uint32 CanalGetVariable(const Res
 	switch (variable) {
 		/* Height of tile */
 		case 0x80: {
-			uint z = GetTileZ(tile);
+			int z = GetTileZ(tile);
 			/* Return consistent height within locks */
 			if (IsTileType(tile, MP_WATER) && IsLock(tile) && GetLockPart(tile) == LOCK_PART_UPPER) z--;
 			return z;