Jmeter 5.4.1 - Groovy script error 'No signature of method'

Newly back to using Jmeter after a 3 year break and pretty confused by this error. My googlefu and programming by Stackoverflow efforts have come to nought, so hopefully someone here will have seen this before.

I have a JSR223 Preprocessor which is used for generating AWS signatures and it keeps generating errors of the following type:

021-10-20 13:47:40,101 ERROR o.a.j.m.JSR223PreProcessor: Problem in JSR223 script, Create AWS Signature 
javax.script.ScriptException: javax.script.ScriptException: groovy.lang.MissingMethodException: No signature of method: is applicable for argument types: () values: []
    at org.codehaus.groovy.jsr223.GroovyScriptEngineImpl.eval( ~[groovy-jsr223-3.0.7.jar:3.0.7]
    at javax.script.AbstractScriptEngine.eval( ~[java.scripting:?]

The AWS signature pre-processor is within the Test Plan area, not the individual thread groups as adding it to individual thread groups seems to have adverse consequences.

It’s not causing the tests themselves to fail (weirdly) but it would be useful to stop these errors clogging our log files.