sum on null object

def customersOnSubbucket;
def problemlt = "17";
def problemportone = "1";
def problemporttwo = "2";
def deviceCustomerCount = [:];


deviceCustomerCount."17-1" = [1,2,3,4,5,6,7,8,9];
//deviceCustomerCount."17-2" = [2,4,6,8,10,12,14,16,18];
print "deviceCustomerCount = " + deviceCustomerCount + "\n";

print "17-1 = " + deviceCustomerCount["17-1"] + "\n";;
print "17-2 = " + deviceCustomerCount["17-2"] + "\n";;

customersOnSubbucket = deviceCustomerCount[problemlt + "-" + problemportone].sum() + deviceCustomerCount[problemlt + "-" + problemporttwo].sum();
print "\n$customersOnSubbucket";