FasterXML/jackson-databind · error · IllegalArgumentException

Cannot pass null Deserializers

Error message

Cannot pass null Deserializers

What it means

Thrown by DeserializerFactoryConfig.withAdditionalDeserializers(Deserializers) when the argument is null. Deserializers providers are the mechanism by which modules and custom code register custom deserializers; a null provider would be silently ignored or cause NPEs later, so it is rejected at registration time.

Solutions

1. Pass a non-null Deserializers instance. If you have nothing to add, do not call the method.
2. Use SimpleDeserializers (from SimpleModule) as a concrete non-null implementation even if empty.
3. Fix the upstream code that produced the null (e.g., a factory method that should return an empty set).

Example fix

// before
Deserializers desers = maybeBuildDeserializers(); // returns null
config.withAdditionalDeserializers(desers); // throws
// after
Deserializers desers = maybeBuildDeserializers();
if (desers != null) {
    config.withAdditionalDeserializers(desers);
}

// Validate before calling
if (deserializers == null) {
    // skip registration or throw a clear error
    throw new IllegalArgumentException("Deserializers provider must not be null");
}
config.withAdditionalDeserializers(deserializers);

boolean isNonNullProvider(Deserializers d) {
    return d != null;
}

When it happens

Trigger: Calling deserializerFactoryConfig.withAdditionalDeserializers(null) directly, or more commonly via SimpleModule.addDeserializer() / SimpleModule.setDeserializerModifier() with internal wiring that resolves to a null Deserializers instance. Also possible via Module.setup() that has a bug returning null.

Common situations: Custom Jackson modules where a factory method or conditional returns null instead of a no-op implementation. Incomplete refactoring where a Deserializers field is left null. Third-party modules with bugs.

Related errors

• Cannot pass null KeyDeserializers
• AnnotationIntrospector returned `Class<
• AnnotationIntrospector.
• AnnotationIntrospector returned Class
• AnnotationIntrospector returned key deserializer definition…

AI-assisted analysis of FasterXML/jackson-databind@87876ca5c0 (2026-08-11). Data as JSON: /api/errors/4ebb38136b10b906. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/tools/jackson/databind/cfg/DeserializerFactoryConfig.java:93

    {
        _additionalDeserializers = (allAdditionalDeserializers == null) ?
                NO_DESERIALIZERS : allAdditionalDeserializers;
        _additionalKeyDeserializers = (allAdditionalKeyDeserializers == null) ?
                DEFAULT_KEY_DESERIALIZERS : allAdditionalKeyDeserializers;
        _modifiers = (modifiers == null) ? NO_MODIFIERS : modifiers;
        _valueInstantiators = (vi == null) ? NO_VALUE_INSTANTIATORS : vi;
    }

    /**
     * Fluent/factory method used to construct a configuration object that
     * has same deserializer providers as this instance, plus one specified
     * as argument. Additional provider will be added before existing ones,
     * meaning it has priority over existing definitions.
     */
    public DeserializerFactoryConfig withAdditionalDeserializers(Deserializers additional)
    {
        if (additional == null) {
            throw new IllegalArgumentException("Cannot pass null Deserializers");
        }
        Deserializers[] all = ArrayBuilders.insertInListNoDup(_additionalDeserializers, additional);
        return new DeserializerFactoryConfig(all, _additionalKeyDeserializers, _modifiers,
                _valueInstantiators);
    }

    /**
     * Fluent/factory method used to construct a configuration object that
     * has same key deserializer providers as this instance, plus one specified
     * as argument. Additional provider will be added before existing ones,
     * meaning it has priority over existing definitions.
     */
    public DeserializerFactoryConfig withAdditionalKeyDeserializers(KeyDeserializers additional)
    {
        if (additional == null) {
            throw new IllegalArgumentException("Cannot pass null KeyDeserializers");
        }
        KeyDeserializers[] all = ArrayBuilders.insertInListNoDup(_additionalKeyDeserializers, additional);

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