Index: main_gui.c
===================================================================
--- main_gui.c  (revision 2886)
+++ main_gui.c  (working copy)
@@ -1219,7 +1219,7 @@
        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 @@
        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);