diff -r 6d64230b9fb9 -r 51bff16a04c9 src/clear_cmd.cpp
--- a/src/clear_cmd.cpp	Thu Dec 18 18:47:39 2008 +0000
+++ b/src/clear_cmd.cpp	Fri Dec 19 01:32:07 2008 +0200
@@ -284,7 +284,7 @@
 		IncreaseGeneratingWorldProgress(GWP_ROUGH_ROCKY);
 		tile = RandomTile();
 		if (IsTileType(tile, MP_CLEAR) && !IsClearGround(tile, CLEAR_DESERT)) SetClearGroundDensity(tile, CLEAR_ROUGH, 3);
-	} while (--i);
+	} while (i && --i);
 
 	/* add rocky tiles */
 	i = gi;
@@ -307,7 +307,7 @@
 			}
 get_out:;
 		}
-	} while (--i);
+	} while (i && --i);
 }
 
 static void ClickTile_Clear(TileIndex tile)