Fix serialization error

Author: julian-elastic

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/enrich/LookupFromIndexService.java

@@ -33,6 +33,7 @@
 import org.elasticsearch.transport.TransportService;
 import org.elasticsearch.xpack.esql.EsqlIllegalArgumentException;
 import org.elasticsearch.xpack.esql.action.EsqlQueryAction;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.expression.NamedExpression;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
@@ -205,6 +206,10 @@ static TransportRequest readFrom(StreamInput in, BlockFactory blockFactory) thro
             List<LookupFromIndexOperator.MatchConfig> matchFields = null;
             if (in.getTransportVersion().onOrAfter(TransportVersions.ESQL_LOOKUP_JOIN_ON_MANY_FIELDS)) {
                 matchFields = in.readCollectionAsList(LookupFromIndexOperator.MatchConfig::new);
+            } else {
+                // For older versions, we only support a single match field.
+                matchFields = new ArrayList<>(1);
+                matchFields.add(new LookupFromIndexOperator.MatchConfig(new FieldAttribute.FieldName(matchField), 0, inputDataType));
             }
             TransportRequest result = new TransportRequest(
                 sessionId,