diff --git a/coordinator/points_writer.go b/coordinator/points_writer.go
index e97c9c2dd2..55fddc4ddb 100644
--- a/coordinator/points_writer.go
+++ b/coordinator/points_writer.go
@@ -255,6 +255,9 @@ func (w *PointsWriter) MapShards(wp *WritePointsRequest) (*ShardMapping, error)
 			mapping.Dropped = append(mapping.Dropped, p)
 			atomic.AddInt64(&w.stats.WriteDropped, 1)
 			continue
+		} else if len(sg.Shards) <= 0 {
+			// Shard groups should have at least one shard.
+			return nil, fmt.Errorf("shard group %d covering %s to %s has no shards", sg.ID, sg.StartTime, sg.EndTime)
 		}
 
 		sh := sg.ShardFor(p)