diff --git a/src/backend/partitioning/partbounds.c b/src/backend/partitioning/partbounds.c
index fdfe712f91..95798f4f66 100644
--- a/src/backend/partitioning/partbounds.c
+++ b/src/backend/partitioning/partbounds.c
@@ -2564,6 +2564,9 @@ build_merged_partition_bounds(char strategy, List *merged_datums,
 		merged_bounds->kind = NULL;
 	}
 
+	/* interleaved_parts is always NULL for join relations. */
+	merged_bounds->interleaved_parts = NULL;
+
 	Assert(list_length(merged_indexes) == ndatums);
 	merged_bounds->nindexes = ndatums;
 	merged_bounds->indexes = (int *) palloc(sizeof(int) * ndatums);
diff --git a/src/include/partitioning/partbounds.h b/src/include/partitioning/partbounds.h
index 9db546def6..7138cb1f2a 100644
--- a/src/include/partitioning/partbounds.h
+++ b/src/include/partitioning/partbounds.h
@@ -72,7 +72,9 @@ struct RelOptInfo;				/* avoid including pathnodes.h here */
  * contain any value that does not belong in another partition.  This field
  * only serves as proof that a particular partition is not interleaved, not
  * proof that it is interleaved.  When we're uncertain, we marked the
- * partition as interleaved.
+ * partition as interleaved.  The interleaved_parts field is only ever set for
+ * RELOPT_BASEREL and RELOPT_OTHER_MEMBER_REL, it is always left NULL for join
+ * relations.
  */
 typedef struct PartitionBoundInfoData
 {