ArrayOutOfBoundsException thrown when labelling a term to access it's attributes - antlr4

Question

I am trying to access the attributes of a rule. Although this dummy rule stub works:
statement returns [List listOfTerms]
: statement booleanOperator statement {$listOfTerms=new ArrayList();}
| delimited {$listOfTerms=getTerms($delimited.text);}
;
This throws an ArrayOutOfBoundsException:
statement returns [List listOfTerms]
: a=statement booleanOperator b=statement {$listOfTerms=new ArrayList();}
| delimited {$listOfTerms=getTerms($delimited.text);}
;
The exception thrown:
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.antlr.v4.parse.GrammarTreeVisitor.visit(GrammarTreeVisitor.java:206)
at org.antlr.v4.parse.GrammarTreeVisitor.visitGrammar(GrammarTreeVisitor.java:200)
at org.antlr.v4.semantics.SymbolCollector.process(SymbolCollector.java:76)
at org.antlr.v4.semantics.SemanticPipeline.process(SemanticPipeline.java:103)
at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:399)
at org.antlr.v4.Tool.process(Tool.java:384)
at org.antlr.mojo.antlr4.Antlr4Mojo$CustomTool.process(Antlr4Mojo.java:461)
at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:343)
at org.antlr.mojo.antlr4.Antlr4Mojo.execute(Antlr4Mojo.java:272)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:320)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:156)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:537)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:196)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:141)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:290)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:230)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:409)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:352)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 2
at org.antlr.v4.semantics.SymbolCollector.discoverOuterAlt(SymbolCollector.java:111)
at org.antlr.v4.parse.GrammarTreeVisitor.outerAlternative(GrammarTreeVisitor.java:2284)
at org.antlr.v4.parse.GrammarTreeVisitor.ruleBlock(GrammarTreeVisitor.java:2197)
at org.antlr.v4.parse.GrammarTreeVisitor.rule(GrammarTreeVisitor.java:1663)
at org.antlr.v4.parse.GrammarTreeVisitor.rules(GrammarTreeVisitor.java:1241)
at org.antlr.v4.parse.GrammarTreeVisitor.grammarSpec(GrammarTreeVisitor.java:480)
... 34 more
This also throws an ArrayoutOfBoundsException:
statement returns [List listOfTerms]
: st+=statement booleanOperator st+=statement {$listOfTerms=new ArrayList();}
| delimited {$listOfTerms=getTerms($delimited.text);}
;
And I get an ISOLATED_RULE_REF if I try to access the statements individually, without labeling them:
errorType=ISOLATED_RULE_REF, args=[statement, $statement]
I have also tried labeling the rule alternatives, but that has no effect.