apache/beam · error · NullPointerException

Null collectionElementType at column

Error message

Null collectionElementType at column 

What it means

In BigtableRowToBeamRow.columnToRow, when a column's Schema.FieldType is declared as a collection (array/iterable), the collection element type must be present. If columnType.getCollectionElementType() returns null, a NullPointerException('Null collectionElementType at column <name>') is thrown. This means the Bigtable schema mapping declared an array/iterable cell type without specifying the element type.

Source

Thrown at sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigtable/BigtableRowToBeamRow.java:120

        rowBuilder.withFieldValue(LABELS, cell.getLabelsList());
      }
      return rowBuilder.build();
    }

    // Returns Simple type, List<Simple type> or Row
    private Object columnToRow(Column column, Schema schema) {
      String columnName = column.getQualifier().toStringUtf8();
      Schema.FieldType columnType = schema.getField(columnName).getType();
      List<Cell> cells = column.getCellsList();
      switch (columnType.getTypeName()) {
        case ARRAY:
          Schema.FieldType collectionElementType = columnType.getCollectionElementType();
          if (collectionElementType != null) {
            return cells.stream()
                .map(cell -> getCellValue(cell, collectionElementType))
                .collect(toList());
          } else {
            throw new NullPointerException("Null collectionElementType at column " + columnName);
          }
        case ROW:
          @Nullable Schema rowSchema = columnType.getRowSchema();
          if (rowSchema == null) {
            throw new NullPointerException("Null row schema at column " + columnName);
          } else {
            return cellToRow(getLastCell(cells), rowSchema);
          }
        default:
          return getCellValue(getLastCell(cells), columnType);
      }
    }

    private Row familyToRow(Family family, Schema schema) {
      Map<String, Object> columns =
          family.getColumnsList().stream()
              .filter(column -> schema.hasField(column.getQualifier().toStringUtf8()))
              .map(

View on GitHub (pinned to 12126d8942)

Solutions

1. Fix the column field type in the Bigtable schema mapping to declare the element type, e.g. Schema.FieldType.array(Schema.FieldType.STRING)
2. If the column is single-valued, map it as a plain type (e.g. STRING/INT64) instead of a collection
3. Regenerate the schema mapping instead of hand-editing serialized Schema JSON
4. Log/validate the schema (print Schema.FieldType) before running the pipeline to confirm collections have component types

Example fix

// before
Schema.FieldType colType = Schema.FieldType.iterable(null);
// after
Schema.FieldType colType = Schema.FieldType.iterable(Schema.FieldType.STRING);

Schema.FieldType t = columnType;
if ((t.getTypeName() == Schema.TypeName.ARRAY || t.getTypeName() == Schema.TypeName.ITERABLE)
    && t.getCollectionElementType() == null) {
  throw new IllegalArgumentException("Column " + columnName + " needs a collection element type");
}

boolean hasElementType(Schema.FieldType t) {
  return t.getCollectionElementType() != null;
}

When it happens

Trigger: Building the BigtableRowDTOWrapper / schema with a column mapped as ARRAY or ITERABLE (component type 'array'/'iterable') but omitting its component element type in the field type definition.

Common situations: Hand-constructed Schema.FieldType for a Bigtable column where multiple cells per row are expected, but the developer wrote FieldType.array(...) incorrectly or built a raw collection type without components; stale serialization of schemas.

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

• Null row schema at column
• Null value at column
• Null family schema at family
• Null column list at family
• Null value found for field that doesn't support nulls.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/e2643c1a3393bb91. Report an issue: GitHub.