VerrifyError for custom metrics

[JuliusKoh]

Hello,

I'm trying to instrument some custom metrics. But I keep running into a java.lang.VerifyError
What I want to achieve: whenever a certain method of a certain class is called, I want to call a getter method of this class and publish the return value (an integer) as metric.
Any ideas, what I am doing wrong?

Here is my configuration:

inspectit:
    exporters:
        metrics:
            prometheus:
                enabled:
                    ENABLED
                port:
                    8889

    service-name:
        'myApp'

    metrics:
        definitions:
            # counts finished tracking steps
            '[webApp/desktops]':
                unit: desktops
                description: 'number of desktops'
                views:
                    '[webApp/desktops/number]':
                        aggregation: LAST_VALUE

  instrumentation:
        scopes:
            's_desktop':
                type:
                    name: 'myApp.SystemMonitor'
                    matcher-mode: EQUALS_FULLY
                methods:
                    - name: 'desktop'
                      matcher-mode: STARTS_WITH

        rules:
            'r_webApp_desktop_number':
                exit: 
                    'current_desktop_number':
                        action: 'a_getActiveDesktopsCount'
                scopes:
                    's_desktop': true
                metrics:
                    '[webApp/desktops]':
                        value: 'current_desktop_number'
                            
                            
        actions:
            'a_getActiveDesktopsCount':
                imports:
                    - myApp
                input:
                    _this: SystemMonitor
                value: _this.getActiveDesktopsCount() 

And this is the Error I get:

2023-04-24 14:23:21,335 ERROR 308671 --- [myApp] [pectit-thread-1] r.i.o.c.i.hook.HookManager               : Error generating hook for desktopCreated(org.zkoss.zk.ui.Desktop) of myApp.SystemMonitor. Method will not be hooked.
java.lang.VerifyError: Bad return type
Exception Details:
  Location:
    myApp/inspectitGen$$$72328829221815119.executeImpl([Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Throwable;[Ljava/lang/Object;)Ljava/lang/Object; @11: ireturn
  Reason:
    Type integer (current frame, stack[0]) is not assignable to 'java/lang/Object' (from method signature)
  Current Frame:
    bci: @11
    flags: { }
    locals: { '[Ljava/lang/Object;', 'java/lang/Object', 'java/lang/Object', 'java/lang/Throwable', '[Ljava/lang/Object;', 'myApp/SystemMonitor' }
    stack: { integer }
  Bytecode:
    0x0000000: 2bc0 0015 3a05 1905 b600 19ac          

	at java.lang.Class.getDeclaredFields0(Native Method)
	at java.lang.Class.privateGetDeclaredFields(Class.java:2583)
	at java.lang.Class.getField0(Class.java:2975)
	at java.lang.Class.getField(Class.java:1701)
	at rocks.inspectit.ocelot.core.instrumentation.actions.bound.BoundGenericAction.<init>(BoundGenericAction.java:77)
	at rocks.inspectit.ocelot.core.instrumentation.actions.bound.ConstantOnlyBoundGenericAction.<init>(ConstantOnlyBoundGenericAction.java:17)
	at rocks.inspectit.ocelot.core.instrumentation.actions.bound.BoundGenericAction.bind(BoundGenericAction.java:63)
	at rocks.inspectit.ocelot.core.instrumentation.hook.ActionCallGenerator.generateAndBindGenericAction(ActionCallGenerator.java:46)
	at rocks.inspectit.ocelot.core.instrumentation.hook.MethodHookGenerator.buildActionCalls(MethodHookGenerator.java:282)
	at rocks.inspectit.ocelot.core.instrumentation.hook.MethodHookGenerator.buildHook(MethodHookGenerator.java:106)
	at rocks.inspectit.ocelot.core.instrumentation.hook.HookManager$HookUpdate.lambda$addOrReplaceHooks$6(HookManager.java:316)
	at java.util.HashMap.forEach(HashMap.java:1290)
	at rocks.inspectit.ocelot.core.instrumentation.hook.HookManager$HookUpdate.addOrReplaceHooks(HookManager.java:307)
	at rocks.inspectit.ocelot.core.instrumentation.hook.HookManager$HookUpdate.updateHooksForClass(HookManager.java:246)
	at rocks.inspectit.ocelot.core.instrumentation.InstrumentationTriggerer.updateClass(InstrumentationTriggerer.java:261)
	at rocks.inspectit.ocelot.core.instrumentation.InstrumentationTriggerer.getBatchOfClassesToRetransform(InstrumentationTriggerer.java:220)
	at rocks.inspectit.ocelot.core.instrumentation.InstrumentationTriggerer.checkClassesForConfigurationUpdates(InstrumentationTriggerer.java:148)
	at rocks.inspectit.ocelot.core.service.BatchJobExecutorService.lambda$startJob$0(BatchJobExecutorService.java:126)
	at rocks.inspectit.ocelot.core.service.BatchJobExecutorService$BatchJob.lambda$new$0(BatchJobExecutorService.java:83)
	at rocks.inspectit.ocelot.core.instrumentation.context.ContextManager.lambda$wrap$1(ContextManager.java:76)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:749)

I would be thankful for some help.