pentaho/pentaho-kettle · error · KettleStepException

Unable to clone row for metadata :

Error message

Unable to clone row for metadata : 

What it means

TransPreviewDelegate registers a RowListener during preview whose rowWrittenEvent clones each incoming row via rowMeta.cloneRow(row) to buffer it for display. If cloning throws, a KettleStepException with 'Unable to clone row for metadata : ' + rowMeta is raised, aborting the preview data flow for that step.

Solutions

1. Inspect the wrapped cause (logged with the exception) to find the field index and mismatch.
2. Fix the producing step so emitted row objects match its RowMetaInterface (correct types per field).
3. Re-run the step's 'Get Fields' so its declared row metadata matches actual output.
4. If a plugin is at fault, update or fix the plugin's processRow implementation.

Example fix

// before: emitting Integer for a declared String field
Object[] row = { Integer.valueOf( 42 ) }; // rowMeta says String -> cloneRow fails
// after
Object[] row = { "42" }; // matches rowMeta String type

try {
  rowsData.add( new RowMetaAndData( rowMeta, rowMeta.cloneRow( row ) ) );
} catch ( Exception e ) {
  log.logError( "Unable to clone row for metadata : " + rowMeta, e );
  // inspect e to find the mismatching field index/type
}

When it happens

Trigger: Previewing a step whose row listener hits the default (non-first/non-last) branch and rowMeta.cloneRow(row) throws — typically when row data does not match the row metadata (wrong object type or null against a non-null field) or the row metadata itself is malformed.

Common situations: Custom step plugins produce rows inconsistent with their declared RowMeta; binary/object fields that cloneRow cannot deep-copy; rows passed after the metadata changed mid-execution.

Related errors

• XMLInputStream.NoIncomingRowsFound
• All fields must have an output type to do the preview
• AnalyticQueryMeta.Exception.SubjectFieldNotFound
• BaseStep.Exception.MetadataDoesntMatchDataRowSize
• BaseStep.SafeMode.Exception.DoubleFieldnames

AI-assisted analysis of pentaho/pentaho-kettle@f3058517a1 (2026-09-13). Data as JSON: /api/errors/855ff72d26b60fcc. Report an issue: GitHub.

Appendix: source

Thrown at ui/src/main/java/org/pentaho/di/ui/spoon/trans/TransPreviewDelegate.java:491

        previewDataMap.put( stepMeta, rowsData );
        previewLogMap.put( stepMeta, loggingText );

        StepInterface step = trans.findRunThread( stepMeta.getName() );

        if ( step != null ) {

          switch ( previewMode ) {
            case LAST:
              step.addRowListener( new RowAdapter() {
                @Override
                public void rowWrittenEvent( RowMetaInterface rowMeta, Object[] row ) throws KettleStepException {
                  try {
                    rowsData.add( new RowMetaAndData( rowMeta, rowMeta.cloneRow( row ) ) );
                    if ( rowsData.size() > PropsUI.getInstance().getDefaultPreviewSize() ) {
                      rowsData.remove( 0 );
                    }
                  } catch ( Exception e ) {
                    throw new KettleStepException( "Unable to clone row for metadata : " + rowMeta, e );
                  }
                }
              } );
              break;
            default:
              step.addRowListener( new RowAdapter() {

                @Override
                public void rowWrittenEvent( RowMetaInterface rowMeta, Object[] row ) throws KettleStepException {
                  if ( rowsData.size() < PropsUI.getInstance().getDefaultPreviewSize() ) {
                    try {
                      rowsData.add( new RowMetaAndData( rowMeta, rowMeta.cloneRow( row ) ) );
                    } catch ( Exception e ) {
                      throw new KettleStepException( "Unable to clone row for metadata : " + rowMeta, e );
                    }
                  }
                }
              } );

View on GitHub (pinned to f3058517a1)