Why do I get "serialization of {...} failed: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)"

This is a common configuration error in groovy when a null value is inserted to a Map.

Our calculation engine does not allow this because such value cannot be persisted to the database or serialized to JSON in HTTP communication.

For example if you try this in Groovy console in Studio:

Ruby

def key = null
return [(key): 1]

it will throw an error:

jakarta.servlet.ServletException: jakarta.servlet.ServletException: com.fasterxml.jackson.databind.JsonMappingException: Null key for a Map not allowed in JSON (use a converting NullKeySerializer?) (through reference chain: java.util.HashMap["response"]->java.util.HashMap["data"]->java.util.ArrayList[0]->net.pricefx.formulaengine.FormulaElementResult["elementResult"]->java.util.LinkedHashMap["null"]) [error:vcg82di5]

This says that the Map in response.data[0].elementResult cannot be serialized to JSON.