diff --git a/docs/multi-stage-query/known-issues.md b/docs/multi-stage-query/known-issues.md
index 8021e1e305b..c76ab57aa7a 100644
--- a/docs/multi-stage-query/known-issues.md
+++ b/docs/multi-stage-query/known-issues.md
@@ -40,7 +40,9 @@ an [UnknownError](./reference.md#error-codes) with a message including "No space
 - SELECT from a Druid datasource does not include unpublished real-time data.
 
 - GROUPING SETS and UNION ALL are not implemented. Queries using these features return a
-  [QueryNotSupported](reference.md#error-codes) error.
+  [QueryNotSupported](reference.md#error-codes) error. 
+
+- For some COUNT DISTINCT queries, you'll encounter a [QueryNotSupported](reference.md#error-codes) error that includes `Must not have 'subtotalsSpec'` as one of its causes. This is caused by the planner attempting to use GROUPING SETs, which are not implemented.
 
 - The numeric varieties of the EARLIEST and LATEST aggregators do not work properly. Attempting to use the numeric
   varieties of these aggregators lead to an error like