diff --git a/core/src/main/java/org/apache/calcite/tools/RelBuilder.java b/core/src/main/java/org/apache/calcite/tools/RelBuilder.java
index ae4e077b97ef..703098cb3457 100644
--- a/core/src/main/java/org/apache/calcite/tools/RelBuilder.java
+++ b/core/src/main/java/org/apache/calcite/tools/RelBuilder.java
@@ -2535,13 +2535,14 @@ private AggregateCall removeRedundantDistinct(
       ImmutableBitSet groupSet,
       RelNode r) {
     if (aggregateCall.isDistinct() && !config.redundantDistinct()) {
-      final RelMetadataQuery mq = r.getCluster().getMetadataQuery();
+      final RelNode relNode = requireNonNull(r, "RelNode");
+      final RelMetadataQuery mq = relNode.getCluster().getMetadataQuery();
       final List<Integer> argList = aggregateCall.getArgList();
       final ImmutableBitSet distinctArg = ImmutableBitSet.builder()
           .addAll(argList)
           .build();
       final ImmutableBitSet columns = groupSet.union(distinctArg);
-      final boolean alreadyUnique = mq.areColumnsUnique(r, columns);
+      final boolean alreadyUnique = mq.areColumnsUnique(relNode, columns);
       if (alreadyUnique) {
         return aggregateCall.withDistinct(false);
       }