Error 000584 when publishing Layer to KML as a web tool with user-defined input mode

[AliAFGHANTOLOEE]

I am trying to publish the Layer to KML tool as a web tool. It works fine when I set the input mode as a choice list. However, when I change the input mode to 'user-defined value' and publish the web tool, I encounter an error after adding a layer and run the tool: ERROR 000584: Implementation of this tool's Validate is invalid. Does anyone know how to resolve this issue?

[XiaoXiao2]

Can you illustrate the workflow for reproducing? Where and how do you run the tool to encounter the error? Thanks