apache/beam · error · IllegalArgumentException

Schema must contain at least one field. Schema: %s

Error message

Schema must contain at least one field. Schema: %s

What it means

PubsubRowToMessage.validate() requires the input Beam Schema to contain at least one field. A zero-field schema cannot be converted into a Pubsub message, so validation throws an IllegalArgumentException including the offending schema.

Source

Thrown at sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubRowToMessage.java:178

  String getAttributesKeyName() {
    return getKeyPrefix() + ATTRIBUTES_KEY_NAME;
  }

  /** Returns the name of the source timestamp key, prefixed with {@link #getKeyPrefix()}. */
  String getSourceEventTimestampKeyName() {
    return getKeyPrefix() + EVENT_TIMESTAMP_KEY_NAME;
  }

  /** Returns the name of the source payload key, prefixed with {@link #getKeyPrefix()}. */
  String getPayloadKeyName() {
    return getKeyPrefix() + PAYLOAD_KEY_NAME;
  }

  /** Validates an input's {@link Schema} for correctness. */
  void validate(Schema schema) {

    if (schema.getFieldCount() == 0) {
      throw new IllegalArgumentException(
          String.format("Schema must contain at least one field. Schema: %s", schema));
    }

    validateAttributesField(schema);
    validateSourceEventTimeStampField(schema);
    validateSerializableFields(schema);
  }

  /**
   * Validates an input's {@link Schema} for its {@link #getAttributesKeyName()} field correctness,
   * if exists.
   */
  void validateAttributesField(Schema schema) {
    String attributesKeyName = getAttributesKeyName();
    if (!schema.hasField(attributesKeyName)) {
      return;
    }
    checkArgument(

View on GitHub (pinned to 12126d8942)

Solutions

1. Add at least one field to the input schema before applying PubsubRowToMessage.
2. Check that the schema source (Avro/JSON schema string, Schema.builder()) actually produced fields.
3. Ensure no upstream transform is stripping all fields from the PCollection's schema.

Example fix

// before
Schema schema = Schema.builder().build(); // 0 fields
// after
Schema schema = Schema.builder().addStringField("payload").build();

if (input.getSchema().getFieldCount() == 0) { throw new IllegalArgumentException("Input schema has no fields; add at least one before PubsubRowToMessage"); }

try { rowToMessage.expand(input); } catch (IllegalArgumentException e) { if (e.getMessage().contains("at least one field")) { /* rebuild schema with fields */ } throw e; }

When it happens

Trigger: Applying PubsubRowToMessage (via expand()) with an input PCollection whose Schema has getFieldCount() == 0 — e.g. an empty schema was parsed or a Row collection with no fields was piped in.

Common situations: Building the schema programmatically and forgetting to add fields; parsing an empty or malformed Avro/JSON schema that yields zero fields; upstream transforms that drop all columns.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• serializable Row does not exist for payload of type: %s
• Unsupported schema specified for Pubsub source in CREATE TAB
• Unsupported schema specified for Pubsub source in CREATE TAB
• Configuration schema provided does not match expected
• Cannot provide a coder for a Beam Row. Please provide a sche

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/ac939c583bd96e80. Report an issue: GitHub.