JavaScript workflow error set state

Hello guys,

I'm trying to achieve the following behavior:

I've got a agile board for conceptionate issues. If the issue gets moved into a column (which is identified by the issuestate), it should disapear from this board and should pop up in the next boards first column.

For this I want to create a custom javascript workflow which triggers on issue change. At this point the issues state should be changed again.

I've got this snippet:

var entities = require('@jetbrains/youtrack-scripting-api/entities');

exports.rule = entities.Issue.onChange({
  // TODO: give the rule a human-readable title
  title: 'Concept-is-done',
  guard: function(ctx) {   
    return ctx.issue.fields.State.name == ctx.State.ConceptDone.name;
  },
  action: function(ctx) {
    var issue = ctx.issue;
    issue.fields.State = ctx.State.ReadyForImplementation;
  },
  requirements: {
    State: {
      type: entities.State.fieldType,
      ConceptDone: {
        name: "Konzeptionierung abgeschlossen"
      },
      ReadyForImplementation: {
          name: "Bereit zur Umsetzung"
      }
    }
  }
});

Sadly, if i try to move the issue in the column "ConceptDone" I get an workflow runtime error with the following error: "Cannot set value to custom field State".

I woudl be happy if someone has an idea.

Best regards