FasterXML/jackson-databind · error · IllegalArgumentException

argument " " is null

Error message

argument "%s" is null

What it means

Thrown by ObjectReader._assertNotNull() when a required non-null argument is passed as null to a public ObjectReader method. This is a fast-fail guard at the API boundary to prevent NullPointerException deeper in the deserialization pipeline where the cause would be unclear.

Solutions

1. Initialize the source variable before calling the reader method.
2. Add a null check in your code before calling: if (input != null) reader.readValue(input).
3. Use Objects.requireNonNull(input, "input") for a clearer error at the call site.

Example fix

// before
String json = mightReturnNull();
MyType result = reader.readValue(json); // throws if json is null
// after
String json = mightReturnNull();
if (json == null) throw new IllegalStateException("source JSON was null");
MyType result = reader.readValue(json);

// Check for null before calling reader
if (source == null) {
    throw new IllegalArgumentException("source must not be null");
}
reader.readValue(source);

When it happens

Trigger: Calling ObjectReader methods like readValue((JsonParser) null), with((InputStream) null), or similar where the internal _assertNotNull check guards the parameter. The paramName in the message identifies which parameter was null.

Common situations: Passing a null InputStream, Reader, File, byte[], or String to readValue/forX methods. Forgetting to initialize a variable before passing it. Conditional code paths that may leave a variable null.

Related errors

• argument " " is null
• maxProblems must be positive
• Cannot pass null Deserializers
• Cannot pass null KeyDeserializers
• Cannot pass null modifier

AI-assisted analysis of FasterXML/jackson-databind@87876ca5c0 (2026-08-11). Data as JSON: /api/errors/df281cda030727c9. Report an issue: GitHub.

Appendix: 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;
        }

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