Can someone tell me why the yearly increase in NWC for 2013 is not 0 but -1000?

Worldwide Paper Company's Invested Capital Requirements Year Net Working Capital Forecast ($000s) Net Working Capital Yearly Increase in NWC Fixed Asset Capital Forecast ($000s) Gross Fixed Assets Net Fixed Assets Total Invested Ca ital 2007 18,000 18,000 18,000 400 400 18,000 16,000 15,400 2 1,000 600 18,000 12,000 13,000 2010 3 1,000 18,000 9,000 10,000 2011 4 1 ,ooo 18,000 6,000 7,000 2012 5 1,000 18,000 3,000 4,000 2013 6 1 ,ooo 18,000 1,000 2014 7