InvokeScriptedProcessor intermittently fails validation

Intermittently (perhaps in the Web Server threads only but perhaps not), InvokeScriptedProcessor will be reported as Invalid with something like the following error logged:

2016-08-25 09:59:46,750 WARN [NiFi Web Server-22] o.a.n.controller.StandardProcessorNode Failed during validation
org.apache.nifi.processor.exception.ProcessException: No script engine available!
         at org.apache.nifi.processors.script.InvokeScriptedProcessor.setup(InvokeScriptedProcessor.java:201) ~[na:na]
         at org.apache.nifi.processors.script.InvokeScriptedProcessor.customValidate(InvokeScriptedProcessor.java:446) ~[na:na]
         at org.apache.nifi.components.AbstractConfigurableComponent.validate(AbstractConfigurableComponent.java:123) ~[nifi-api-1.0.0-SNAPSHOT.jar:1.0.0-SNAPSHOT]
         at org.apache.nifi.controller.StandardProcessorNode.isValid(StandardProcessorNode.java:917) ~[nifi-framework-core-1.0.0-SNAPSHOT.jar:1.0.0-SNAPSHOT]
         at org.apache.nifi.controller.FlowController.getProcessorStatus(FlowController.java:2710) [nifi-framework-core-1.0.0-SNAPSHOT.jar:1.0.0-SNAPSHOT]