What causes "Param was null" in quarkusio/quarkus?

Ensure a non-null MediaType is always produced when the response/entity is built, defaulting to APPLICATION_OCTET_STREAM for unknown types.

quarkusio/quarkus · error · IllegalArgumentException

Param was null

Error message

Param was null

What it means

MediaTypeHeaderDelegate.toString converts a MediaType object back to its header string form and rejects a null argument per the HeaderDelegate contract. Since serialization of a null header value is undefined, the delegate throws IllegalArgumentException up front rather than emitting 'null'.

Source

Thrown at independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/MediaTypeHeaderDelegate.java:133

        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            for (char q : quotedChars) {
                if (c == q)
                    return true;
            }
        }
        return false;
    }

    public MediaType fromString(String type) throws IllegalArgumentException {
        if (type == null)
            throw new IllegalArgumentException("Media type was null");
        return parse(type);
    }

    public String toString(MediaType o) {
        if (o == null)
            throw new IllegalArgumentException("Param was null");
        MediaType type = o;
        String result = reverseMap.get(type);
        if (result == null) {
            result = internalToString(type);
            final int size = reverseMap.size();
            if (size >= MAX_MT_CACHE_SIZE) {
                reverseMap.clear();
                map.clear();
            }
            reverseMap.put(type, result);
            map.put(result, type);
        }
        return result;
    }

    private static String internalToString(MediaType type) {
        StringBuilder buf = new StringBuilder(type.getType().length() + type.getSubtype().length() + 1);

View on GitHub (pinned to e1c734241f)

Solutions

Ensure a non-null MediaType is always produced when the response/entity is built, defaulting to APPLICATION_OCTET_STREAM for unknown types.
Add an explicit null check at the call site and skip or default the header instead of serializing null.
Catch IllegalArgumentException at the header-writing boundary as a defensive guard.

Example fix

// before
builder.header(HttpHeaders.CONTENT_TYPE, computedType); // computedType may be null
// after
builder.header(HttpHeaders.CONTENT_TYPE, computedType != null ? computedType : MediaType.APPLICATION_OCTET_STREAM);

Defensive patterns

Strategy: type-guard

Validate before calling

Objects.requireNonNull(computedType, "Content-Type must be resolved before building the response");

Type guard

static boolean isSerializable(MediaType mt) {
    return mt != null;
}

Try / catch

try {
    String header = delegate.toString(mediaType);
} catch (IllegalArgumentException e) {
    // null media type: skip or use default
}

Prevention

Initialize MediaType fields eagerly; never leave them null on response paths.
Resolve a default Content-Type when the request did not provide one.
Assert non-null media types in unit tests for response-building code.
Avoid generic header-writing utilities that accept nullable values.

When it happens

Trigger: Calling MediaTypeHeaderDelegate.toString(null), or letting a null MediaType flow into header serialization (e.g. setting a Response header/Content-Type from a variable that is null).

Common situations: Building a Response where Content-Type is computed from a parsed request value that was never set; caching code storing null media types; generic header-writing utilities receiving null objects.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/573f8931e89d9733. Report an issue: GitHub.