diff --git a/tsdb/index/inmem/inmem.go b/tsdb/index/inmem/inmem.go
index e257b8214c..6f34dfe434 100644
--- a/tsdb/index/inmem/inmem.go
+++ b/tsdb/index/inmem/inmem.go
@@ -1066,13 +1066,11 @@ func (i *Index) assignExistingSeries(shardID uint64, seriesIDSet *tsdb.SeriesIDS
 		} else {
 			// Add the existing series to this shard's bitset, since this may
 			// be the first time the series is added to this shard.
-			if !seriesIDSet.Contains(ss.ID) {
-				seriesIDSet.Lock()
-				if !seriesIDSet.ContainsNoLock(ss.ID) {
-					seriesIDSet.AddNoLock(ss.ID)
-				}
-				seriesIDSet.Unlock()
+			seriesIDSet.Lock()
+			if !seriesIDSet.ContainsNoLock(ss.ID) {
+				seriesIDSet.AddNoLock(ss.ID)
 			}
+			seriesIDSet.Unlock()
 		}
 	}
 	i.mu.RUnlock()