diff --git a/extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregator.java b/extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregator.java
index 6b73390c192..b7df576e949 100644
--- a/extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregator.java
+++ b/extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregator.java
@@ -122,7 +122,9 @@ public class SketchAggregator implements Aggregator
       union.update((long[]) update);
     } else if (update instanceof List) {
       for (Object entry : (List) update) {
-        union.update(entry.toString());
+        if (entry != null) {
+          union.update(entry.toString());
+        }
       }
     } else {
       throw new ISE("Illegal type received while theta sketch merging [%s]", update.getClass());
diff --git a/extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregationTest.java b/extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregationTest.java
index 19716d968e6..3e7d47d8b07 100644
--- a/extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregationTest.java
+++ b/extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregationTest.java
@@ -35,8 +35,10 @@ import org.apache.druid.java.util.common.granularity.Granularities;
 import org.apache.druid.java.util.common.guava.Sequence;
 import org.apache.druid.query.Query;
 import org.apache.druid.query.aggregation.AggregationTestHelper;
+import org.apache.druid.query.aggregation.Aggregator;
 import org.apache.druid.query.aggregation.AggregatorFactory;
 import org.apache.druid.query.aggregation.PostAggregator;
+import org.apache.druid.query.aggregation.TestObjectColumnSelector;
 import org.apache.druid.query.aggregation.post.FieldAccessPostAggregator;
 import org.apache.druid.query.groupby.GroupByQuery;
 import org.apache.druid.query.groupby.GroupByQueryConfig;
@@ -493,6 +495,25 @@ public class SketchAggregationTest
     Assert.assertEquals(holders[0].getEstimate(), holders[1].getEstimate(), 0);
   }
 
+  @Test
+  public void testUpdateUnionWithNullInList()
+  {
+    List<String> value = new ArrayList<>();
+    value.add("foo");
+    value.add(null);
+    value.add("bar");
+    List[] columnValues = new List[]{value};
+    final TestObjectColumnSelector selector = new TestObjectColumnSelector(columnValues);
+    final Aggregator agg = new SketchAggregator(selector, 4096);
+    agg.aggregate();
+    Assert.assertFalse(agg.isNull());
+    Assert.assertNotNull(agg.get());
+    Assert.assertTrue(agg.get() instanceof SketchHolder);
+    Assert.assertEquals(2, ((SketchHolder) agg.get()).getEstimate(), 0);
+    Assert.assertNotNull(((SketchHolder) agg.get()).getSketch());
+    Assert.assertEquals(2, ((SketchHolder) agg.get()).getSketch().getEstimate(), 0);
+  }
+
   private void assertPostAggregatorSerde(PostAggregator agg) throws Exception
   {
     Assert.assertEquals(