jBPM process instance variables getting set to null after setting a process instance key

Issue

We create a new jBPM process using below constructor:

org.jbpm.graph.exe.ProcessInstance.ProcessInstance(ProcessDefinition processDefinition, Map variables);

At this point all the parameters passed to the process are stored in contextInstance:

processInstance.getContextInstance().getVariables().get("paramName"); 

returns the value of the parameter.

But when we set the key in the process instance:

processInstance.setKey("a_key");

then the variableMap is reset to null i.e. processInstance.getContextInstance().getVariables() returns null.