ROS_ASSERT fails, ros parameter type invalid

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I've been trying to load yaml files to the parameter server and process them with a C++ program. What I've found is that, even with simple yaml files such as:

chains: [5,6,7]

the lines of code:

XmlRpc::XmlRpcValue chains;
  nh.getParam("chains", chains);
  std::cout << chains.getType() << std::endl;

outputs a 0, which means the type is

XmlRpc::XmlRpcValue::TypeInvalid

I'm not exactly surewhy this is happening regardless of the parameter I place on the server... but I'm out of ideas.

Any help would be appreciated.