diff --git a/processing/src/main/java/io/druid/query/aggregation/cardinality/CardinalityAggregator.java b/processing/src/main/java/io/druid/query/aggregation/cardinality/CardinalityAggregator.java
index ae96ff13736b..f79717eca817 100644
--- a/processing/src/main/java/io/druid/query/aggregation/cardinality/CardinalityAggregator.java
+++ b/processing/src/main/java/io/druid/query/aggregation/cardinality/CardinalityAggregator.java
@@ -109,7 +109,9 @@ public void reset()
   @Override
   public Object get()
   {
-    return collector;
+    // Workaround for non-thread-safe use of HyperLogLogCollector.
+    // OnheapIncrementalIndex has a penchant for calling "aggregate" and "get" simultaneously.
+    return HyperLogLogCollector.makeCollectorSharingStorage(collector);
   }
 
   @Override
diff --git a/processing/src/main/java/io/druid/query/aggregation/hyperloglog/HyperUniquesAggregator.java b/processing/src/main/java/io/druid/query/aggregation/hyperloglog/HyperUniquesAggregator.java
index 5ca26a7552eb..caf3476c0658 100644
--- a/processing/src/main/java/io/druid/query/aggregation/hyperloglog/HyperUniquesAggregator.java
+++ b/processing/src/main/java/io/druid/query/aggregation/hyperloglog/HyperUniquesAggregator.java
@@ -55,7 +55,8 @@ public void reset()
   @Override
   public Object get()
   {
-    // Workaround for OnheapIncrementalIndex's penchant for calling "aggregate" and "get" simultaneously.
+    // Workaround for non-thread-safe use of HyperLogLogCollector.
+    // OnheapIncrementalIndex has a penchant for calling "aggregate" and "get" simultaneously.
     return HyperLogLogCollector.makeCollectorSharingStorage(collector);
   }