RRD shows unknown data in the graph

Hi,

I'm using RRD tool to monitor a group of hosts. We have scripts to fetch the time series data from all the hosts and that data is stored in an Oracle DB.
And for every hour this time series data is transfered from Oracle DB to RRD database through a perl script. It's been working fine since long time.

Now I have discovered a problem with values supposed to be unknown ( Channels not being updated because the corresponding hosts are down for 3 hours):
But they don't show up as unknown/NaN in the database - instead it show a constant value let's say 150 in the graphs during that 3 hrs down time.

Can any one help how do I track this problem, it's very urgent.

Thanks in advance.