What causes "argument "%s" is null" in FasterXML/jackson-databind?

Check for null before calling read methods: Objects.requireNonNull(src, "src") or an explicit if/null branch.

FasterXML/jackson-databind · error · IllegalArgumentException

argument "%s" is null

Error message

argument "%s" is null

What it means

ObjectReader._assertNotNull() is a precondition guard invoked by nearly every read/forType/readValue/readTree/etc. entry point (57 call sites) to fail fast with a clear message when a required argument is null. The message includes the parameter name (e.g. "src", "content", "p", "type") so you know exactly which argument was null. It replaces a latent NullPointerException deeper in the call with an explicit, descriptive IllegalArgumentException at the API boundary.

Source

Thrown at src/main/java/tools/jackson/databind/ObjectReader.java:2115

    }

    /**
     * Internal helper method called to create an instance of {@link DeserializationContext}
     * for deserializing a single root value.
     * Can be overridden if a custom context is needed.
     */
    protected DeserializationContextExt _deserializationContext() {
        return _contexts.createContext(_config, _schema, _injectableValues);
    }

    protected DeserializationContextExt _deserializationContext(JsonParser p) {
        return _contexts.createContext(_config, _schema, _injectableValues)
                .assignParser(p);
    }

    protected final void _assertNotNull(String paramName, Object src) {
        if (src == null){
            throw new IllegalArgumentException("argument \"%s\" is null".formatted(paramName));
        }
    }

    /*
    /**********************************************************************
    /* Helper methods, locating deserializers etc
    /**********************************************************************
     */

    /**
     * Method called to locate deserializer for the passed root-level value.
     */
    protected ValueDeserializer<Object> _findRootDeserializer(DeserializationContext ctxt)
        throws DatabindException
    {
        if (_rootDeserializer != null) {
            return _rootDeserializer;
        }

View on GitHub (pinned to a50c7d2a1d)

Solutions

Check for null before calling read methods: Objects.requireNonNull(src, "src") or an explicit if/null branch.
If null is legitimately 'no input', handle it at the caller with an empty/Optional/default rather than passing it down.
Annotate the producing method/field with @NotNull / @NonNull and enable static null-analysis (Checker Framework, IntelliJ nullability, Eclipse null annotations).
Where the value comes from a Map or external source, use getOrDefault / Optional.ofNullable to guarantee non-null.

Example fix

// before
String body = request.getBody(); // may be null
MyType v = reader.readValue(body); // throws: argument "src" is null
// after
String body = request.getBody();
if (body == null) throw new BadRequestException("body required");
MyType v = reader.readValue(body);

Defensive patterns

Strategy: validation

Validate before calling

String body = ...;
if (body == null) throw new IllegalArgumentException("src must not be null");
reader.readValue(body);

Type guard

// simple non-null check is the guard; for collections, check emptiness too

Try / catch

try {
    return reader.readValue(src);
} catch (IllegalArgumentException e) {
    if (e.getMessage().endsWith("is null")) {
        // provide a domain-meaningful default or 4xx response
    }
    throw e;
}

Prevention

Null-check at the controller/service boundary before calling read methods.
Use Optional and serialize only when present.
Enable static null analysis (@NonNull annotations) to catch null flows at compile time.
Prefer Objects.requireNonNull(src, "src") for clear precondition failures.

When it happens

Trigger: Calling reader.readValue((String)null), reader.readTree((JsonParser)null), reader.forType((Class)null), reader.readValues((InputStream)null), etc. Common when the source variable comes from a map.get() that returned null, a resource lookup that failed, or a generically-typed parameter that the caller left null.

Common situations: A field/property that was optional in the source data but is passed unchecked; a Spring/Hazelcast/JNDI resource lookup returning null on misconfiguration and being forwarded directly; HTTP request body or path variable that is null when the endpoint receives no body; refactoring that moved a null check away from the call site.

Related errors

AI-assisted analysis of FasterXML/jackson-databind@a50c7d2a1d (2026-08-06). Data as JSON: /api/errors/df281cda030727c9. Report an issue: GitHub.