What causes "Must set either comment or data" in quarkusio/quarkus?

Call data(...) with a non-null payload before build()

quarkusio/quarkus · error · IllegalArgumentException

Must set either comment or data

Error message

Must set either comment or data

What it means

OutboundSseEventImpl.Builder.build() validates that an outbound SSE event carries content: at least one of a comment or data must be set. An event with neither would serialize to an empty/invalid SSE frame, so IllegalArgumentException is thrown.

Source

Thrown at independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/jaxrs/OutboundSseEventImpl.java:201

        public BuilderImpl data(Object data) {
            Objects.requireNonNull(data);

            if (data instanceof GenericEntity) {
                GenericEntity<?> genericEntity = (GenericEntity<?>) data;
                this.type = genericEntity.getRawType();
                this.genericType = genericEntity.getType();
                this.data = genericEntity.getEntity();
            } else {
                data(data.getClass(), data);
            }

            return this;
        }

        @Override
        public OutboundSseEventImpl build() {
            if (this.comment == null && this.data == null) {
                throw new IllegalArgumentException("Must set either comment or data");
            }
            return new OutboundSseEventImpl(name, id, reconnectDelay, type, genericType, mediaType, data, comment);
        }
    }
}

View on GitHub (pinned to e1c734241f)

Solutions

Call data(...) with a non-null payload before build()
Or call comment(...) for keep-alive/annotation-only events
Skip emitting the event entirely when there is no content
If null data is legitimate, convert it to an explicit empty string or placeholder

Example fix

// before
OutboundSseEvent evt = sse.newEventBuilder()
    .name("update")
    .build(); // no comment or data
// after
OutboundSseEvent evt = sse.newEventBuilder()
    .name("update")
    .data(payload != null ? payload : "")
    .build();

Defensive patterns

Strategy: validation

Validate before calling

if (eventData == null && eventComment == null) {
    throw new IllegalArgumentException("SSE event needs comment or data");
}

Try / catch

try { return builder.build(); } catch (IllegalArgumentException e) { if (e.getMessage().equals("Must set either comment or data")) { return builder.data("").build(); } throw e; }

Prevention

Always set data(...) or comment(...) before build()
Skip event emission when payload is absent
Default null payloads to an explicit empty string or sentinel

When it happens

Trigger: Building an OutboundSseEvent via newEventBuilder() and calling build() without calling either comment(...) or data(...) — e.g. only setting name/id/reconnectDelay, or conditionally skipping data and leaving it unset.

Common situations: Dynamically built SSE events where a null entity silently skips data(...); refactors that renamed data-setting methods; mapping code that filters out payloads but still emits an event object.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/b98add8a3c69d9d4. Report an issue: GitHub.