Network Topology Strategy error

[A J]

using latest cassandra (0.7.2). I want to try out Network Topology Strategy.

Following is related setting in cassandra.yaml
endpoint_snitch: org.apache.cassandra.locator.PropertyFileSnitch

I have four nodes. Set them accordingly in ./conf/cassandra-topology.properties:
10.252.219.224=DC2:RAC1
10.252.10.64=DC2:RAC1
10.252.11.32=DC1:RAC1
10.220.103.98=DC1:RAC1


I create a ks as:
create keyspace ks1 with replication_factor=1 and
placement_strategy='org.apache.cassandra.locator.NetworkTopologyStrategy';

When I try to insert, get the following error:
set cf1['A']['c1']='xyz';
ERROR 19:21:58,081 Internal error processing insert
java.lang.AssertionError: invalid response count 1 for replication factor 0


Please suggest what could be going on ? cassandra-topology.properties
has two DCs. Why am I still getting the error ?

Thanks for any suggestions.