diff --git a/main_gui.c b/main_gui.c
--- a/main_gui.c
+++ b/main_gui.c
@@ -1219,7 +1219,7 @@ static void PlaceProc_LightHouse(TileInd
 	TileInfo ti;
 
 	FindLandscapeHeightByTile(&ti, tile);
-	if (IsTileType(tile, MP_CLEAR) || IsSteepTileh(ti.tileh))
+	if (!IsTileType(tile, MP_CLEAR) || IsSteepTileh(ti.tileh))
 		return;
 
 	ModifyTile(tile, MP_SETTYPE(MP_UNMOVABLE) | MP_MAP5, 1);
@@ -1231,7 +1231,7 @@ static void PlaceProc_Transmitter(TileIn
 	TileInfo ti;
 
 	FindLandscapeHeightByTile(&ti, tile);
-	if (IsTileType(tile, MP_CLEAR) || IsSteepTileh(ti.tileh))
+	if (!IsTileType(tile, MP_CLEAR) || IsSteepTileh(ti.tileh))
 		return;
 
 	ModifyTile(tile, MP_SETTYPE(MP_UNMOVABLE) | MP_MAP5, 0);