I have a process with a receive event task and a timer boundary event. Using the REST API I try to signal the execution, however I get a null pointer exception. Partial stack trace below...

java.lang.NullPointerException
org.camunda.bpm.engine.impl.persistence.entity.ExecutionEntity.signal(ExecutionEntity.java:376)
org.camunda.bpm.engine.impl.cmd.SignalCmd.execute(SignalCmd.java:71)
org.camunda.bpm.engine.impl.interceptor.CommandExecutorImpl.execute(CommandExecutorImpl.java:24)
org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:42)
org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:32)
org.camunda.bpm.engine.impl.RuntimeServiceImpl.signal(RuntimeServiceImpl.java:202)
org.camunda.bpm.engine.rest.sub.runtime.impl.ExecutionResourceImpl.signalExecution(ExecutionResourceImpl.java:60)....