apache/druid · error · org.apache.druid.java.util.common.ISE

Value is not an array, got

Error message

Value is not an array, got [%s] instead

What it means

This error is thrown by VariantArrayFieldColumnWriter.processValue when a value passed to the nested-field array writer is not an Object[]. The writer is only used for array-typed fields, and upstream coercion is supposed to guarantee that every non-null value is an Object[] before reaching this point; encountering anything else means an internal type invariant was violated.

Solutions

1. Inspect the input data at the failing row and confirm the array field actually contains arrays at ingestion
2. Ensure values pass through the standard nested-field coercion path (FieldTypeInfo/coercion to Object[]) before writing
3. Upgrade Druid to a version with the latest nested-column coercion fixes
4. File a bug with the failing input row if standard ingestion triggers this

Example fix

// before
writer.processValue(someScalar);
// after
if (!(value instanceof Object[])) { value = value == null ? null : new Object[]{value}; }
writer.processValue((Object[]) value);

if (value != null && !(value instanceof Object[])) { throw new IllegalArgumentException("expected array, got " + value.getClass()); }

static boolean isArrayValue(Object v) { return v instanceof Object[]; }

try { writer.processValue(v); } catch (IllegalStateException e) { if (e.getMessage().startsWith("Value is not an array")) { /* coerce or skip row */ } else { throw e; } }

When it happens

Trigger: Writing a nested variant column where a row value for an array field was not coerced to Object[] (e.g. a scalar or null-wrapped value slipped through the coercion in the nested field indexer), or calling processValue directly with a non-array value.

Common situations: Custom or patched ingestion code feeding raw values into the nested column writer; Druid version changes where coercion behavior for mixed-type array fields changed; segment generation during ingestion of JSON with an array field whose elements were flattened or mistyped.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• attempt to get boolean[] null vector from string[] only…
• attempt to get double[] from string[] only scalar binding
• attempt to get long[] from string[] only scalar binding
• Cannot call makeDimensionSelector on field of type
• Cannot cast [ ] to [ ] (Types.InvalidCastException from…

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/f63d68fdfb787140. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/segment/nested/VariantArrayFieldColumnWriter.java:73

          globalIds[i] = globalDictionaryIdLookup.lookupString((String) array[i]);
        } else if (array[i] instanceof Long) {
          globalIds[i] = globalDictionaryIdLookup.lookupLong((Long) array[i]);
        } else if (array[i] instanceof Double) {
          globalIds[i] = globalDictionaryIdLookup.lookupDouble((Double) array[i]);
        } else {
          globalIds[i] = -1;
        }
        Preconditions.checkArgument(globalIds[i] >= 0, "unknown global id [%s] for value [%s]", globalIds[i], array[i]);
        arrayElements.computeIfAbsent(
            globalIds[i],
            (id) -> columnFormatSpec.getBitmapEncoding().getBitmapFactory().makeEmptyMutableBitmap()
        ).add(row);
      }
      return globalIds;
    }
    if (value != null) {
      // this writer is only used for arrays so all values will have been coerced to Object[] at this point
      throw new ISE("Value is not an array, got [%s] instead", value.getClass());
    }
    return null;
  }

  @Override
  int lookupGlobalId(int[] value)
  {
    return globalDictionaryIdLookup.lookupArray(value);
  }
}

View on GitHub (pinned to 9b90983fd2)