parboiled: Illegal rule definition: Unwrapped action expression!

ExampleGroovyParser.groovy

import org.parboiled.BaseParser
import org.parboiled.Parboiled
import org.parboiled.Rule
import org.parboiled.annotations.BuildParseTree
import org.parboiled.parserunners.ReportingParseRunner
import org.parboiled.support.ParseTreeUtils

@BuildParseTree
class ExampleGroovyParser extends BaseParser {
    // start = "a"+ "b"+
    Rule start() { Sequence(OneOrMore(String("a")), 
                   OneOrMore(String("b")), push("fin")) }
    
    static void main(String[] args) {
        ExampleGroovyParser parser = Parboiled.createParser(ExampleGroovyParser)
        def parseRunner = new ReportingParseRunner(parser.start())
        
        def input = "aabbb"
        def result = parseRunner.run(input)
        println ParseTreeUtils.printNodeTree(result)
    }
}

ExampleJavaParser.java

import org.parboiled.BaseParser;
import org.parboiled.Parboiled;
import org.parboiled.Rule;
import org.parboiled.annotations.BuildParseTree;
import org.parboiled.parserunners.ReportingParseRunner;
import org.parboiled.support.ParseTreeUtils;
import org.parboiled.support.ParsingResult;

@BuildParseTree
public class ExampleJavaParser extends BaseParser<Object> {
    // start = "a"+ "b"+
    Rule start() { return Sequence(OneOrMore(String("a")), 
                   OneOrMore(String("b")), push("fin")); }
    
    @SuppressWarnings({ "rawtypes", "unchecked" })
    public static void main(String[] args) {
        ExampleJavaParser parser = Parboiled.createParser(ExampleJavaParser.class);
        ReportingParseRunner parseRunner = new ReportingParseRunner(parser.start());
        
        String input = "aabbb";
        ParsingResult result = parseRunner.run(input);
        System.out.println(ParseTreeUtils.printNodeTree(result));
    }
}

grammar-exception.txt

Exception in thread "main" org.parboiled.errors.GrammarException: Illegal rule definition: Unwrapped action expression!
    at org.parboiled.support.Checks.ensure(Checks.java:49)
    at org.parboiled.BaseActions.check(BaseActions.java:435)
    at org.parboiled.BaseActions.push(BaseActions.java:172)
    at org.parboiled.BaseActions$push.callCurrent(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:44)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:141)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:149)
    at ExampleGroovyParser.start(ExampleGroovyParser.groovy:12)
    at ExampleGroovyParser$$parboiled.start(Unknown Source)
    at ExampleGroovyParser$$parboiled$start.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:40)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:116)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:120)
    at ExampleGroovyParser.main(ExampleGroovyParser.groovy:16)