Cacti Always Show NaN Value

[darkswar]

I managed to make a graph. and RRD files have been made​​, but why my cacti just shows NaN values​​?

root@cacti:/usr/share/cacti/site/rra# rrdtool fetch oixp_-_core_-_traffic_in_661.rrd AVERAGE | tail -f
1411835760: -nan -nan
1411836120: -nan -nan
1411836480: -nan -nan
1411836840: -nan -nan
1411837200: -nan -nan
1411837560: -nan -nan
1411837920: -nan -nan
1411838280: -nan -nan
1411838640: -nan -nan
1411839000: -nan -nan

Please help me

[cigamit]

Check the cacti log, see what results are being returned for this data source.