diff --git a/tsdb/index/postings.go b/tsdb/index/postings.go
index 8ca0ee605..6cdfa56b1 100644
--- a/tsdb/index/postings.go
+++ b/tsdb/index/postings.go
@@ -419,6 +419,7 @@ func (p *MemPostings) PostingsForLabelMatching(ctx context.Context, name string,
 	count := 1
 	for _, v := range vals {
 		if count%checkContextEveryNIterations == 0 && ctx.Err() != nil {
+			p.mtx.RUnlock()
 			return ErrPostings(ctx.Err())
 		}
 		count++