case management - task boundary timers error at runtime

when the timer fires there is an exception at runtime:

10:26:58,370 ERROR [org.jbpm.process.instance.timer.TimerManager] (EJB default - 8) Error when executing timer job: org.jbpm.workflow.instance.WorkflowRuntimeException: [case-test.sample-case:2 - Task 1:10] – null
at org.jbpm.workflow.instance.impl.NodeInstanceImpl.executeAction(NodeInstanceImpl.java:230)
at org.jbpm.workflow.instance.node.StateBasedNodeInstance.triggerTimer(StateBasedNodeInstance.java:345)
at org.jbpm.workflow.instance.node.StateBasedNodeInstance.signalEvent(StateBasedNodeInstance.java:324)
(...)

Caused by: java.lang.NullPointerException
at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
at java.util.regex.Matcher.reset(Matcher.java:309)
at java.util.regex.Matcher.<init>(Matcher.java:229)
at java.util.regex.Pattern.matcher(Pattern.java:1093)
at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.resolveVariable(WorkflowProcessInstanceImpl.java:712)
at org.jbpm.workflow.instance.impl.WorkflowProcessInstanceImpl.lambda$getResolver$0(WorkflowProcessInstanceImpl.java:677)
at org.jbpm.workflow.core.node.DynamicNode.acceptsEvent(DynamicNode.java:43)