avoid segfault when no matching chromosome can be found
avoid segfault when no matching chromosome can be found

bmvdgeijn/WASP,gmcvicker/WASP,gmcvicker/WASP,bmvdgeijn/WASP,gmcvicker/WASP,bmvdgeijn/WASP,bmvdgeijn/WASP,gmcvicker/WASP