diff --git a/pravega-sensor-collector/src/main/java/io/pravega/sensor/collector/file/parquet/ParquetEventGenerator.java b/pravega-sensor-collector/src/main/java/io/pravega/sensor/collector/file/parquet/ParquetEventGenerator.java
index 3b9c2c16..8617e470 100644
--- a/pravega-sensor-collector/src/main/java/io/pravega/sensor/collector/file/parquet/ParquetEventGenerator.java
+++ b/pravega-sensor-collector/src/main/java/io/pravega/sensor/collector/file/parquet/ParquetEventGenerator.java
@@ -57,7 +57,7 @@ public class ParquetEventGenerator implements EventGenerator {
     private final int maxRecordsPerEvent;
     private final ObjectNode eventTemplate;
     private final ObjectMapper mapper;
-    private Map<MessageType, Schema> parquetSchemas = new HashMap<>();
+    private final Map<MessageType, Schema> parquetSchemas = new HashMap<>();
 
     public ParquetEventGenerator(String routingKey, int maxRecordsPerEvent, ObjectNode eventTemplate, ObjectMapper mapper) {
         this.routingKey = Preconditions.checkNotNull(routingKey, "routingKey");