JsonSerialize` with `nullUsing` option not working for `String

Hi All, if i use AfterModule this.registerModule(new AfterburnerModule()); the following annotation @JsonSerialize(nullsUsing=CustomSerialize.class) not work. How can enable it ? i want to customize null serialization for certain fields.

Error: The JSON value could not be converted to System.Collections.Generic.IEnumerable`1[Newtonsoft.Json.Linq.JToken]. Path: $.variables, LineNumber: 0

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Error: The JSON value could not be converted to System.Collections.Generic.IEnumerable`1[Newtonsoft.Json.Linq.JToken]. Path: $.variables, LineNumber: 0

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