# Unknown method used in expression

**URL:** <https://forum.flowable.org/t/unknown-method-used-in-expression/10439>\
**Category:** Flowable Engine\
**Created:** [April 20, 2023, 12:08pm UTC](https://forum.flowable.org/t/unknown-method-used-in-expression/10439 "2023-04-20T12:08:07Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![kaschnic](https://avatars.discourse-cdn.com/v4/letter/k/8491ac/32.png) [@kaschnic](https://forum.flowable.org/u/kaschnic)\
**Post date:** [April 20, 2023, 12:08pm UTC](https://forum.flowable.org/t/unknown-method-used-in-expression/10439/1 "2023-04-20T12:08:07Z")

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Hi there,

our Workflow ist struggling with an expression an a serviceTask.

The expression says ${error\_list.put(cause\_application\_name, messages\_array)}

error\_list is a JSON (set via initializa variables task)  
cause\_application\_name is a fixed string e.g. invoice\_tracker  
messages\_array is a variable, that looks up the JSON message send via Kafka ${reply\_body\_invoice\_tracker.errors}

We are using this service task with the very same expression more than once. But only one service task reports the error-log: “Unknown method used in expression ${error\_list.put(cause\_application\_name, messages\_array)}”

Why is it, that this expression works just fine for other parts in the workflow?

Or what exactly dose “Unknown method used” mean?

Thanks for your help.  
Nicole

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**Author:** ![WelschChristopher](https://yyz1.discourse-cdn.com/flex035/user_avatar/forum.flowable.org/welschchristopher/32/2822_2.png) [@WelschChristopher](https://forum.flowable.org/u/WelschChristopher)\
**Post date:** [April 20, 2023, 12:15pm UTC](https://forum.flowable.org/t/unknown-method-used-in-expression/10439/2 "2023-04-20T12:15:17Z")

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Hey @kaschnic,  
your question seems to be flowable enterprise related. Please use the enterprise forum to ask your question.

Greetings

Christopher

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**Author:** ![kaschnic](https://avatars.discourse-cdn.com/v4/letter/k/8491ac/32.png) [@kaschnic](https://forum.flowable.org/u/kaschnic)\
**Post date:** [April 20, 2023, 12:27pm UTC](https://forum.flowable.org/t/unknown-method-used-in-expression/10439/3 "2023-04-20T12:27:22Z")

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Hi @WelschChristopher ,

allright, thanks and sorry, I think i mixed something up.

Kind rigards  
Nicole