diff --git a/tsdb/index/postings.go b/tsdb/index/postings.go
index 3256ca55b..ad76f647f 100644
--- a/tsdb/index/postings.go
+++ b/tsdb/index/postings.go
@@ -855,17 +855,22 @@ func (it *ListPostings) Next() bool {
 
 func (it *ListPostings) Seek(x storage.SeriesRef) bool {
 	// If the current value satisfies, then return.
-	if it.list[it.i] >= x {
+	if it.i >= 0 && it.list[it.i] >= x {
 		return true
 	}
 
+	start := it.i
+	if start < 0 {
+		start = 0
+	}
+
 	// Do binary search between current position and end.
-	l := it.list[it.i:]
+	l := it.list[start:]
 	i := sort.Search(len(l), func(i int) bool {
 		return l[i] >= x
 	})
 	if i < len(l) {
-		it.i += i
+		it.i = start + i
 		return true
 	}
 
@@ -921,7 +926,12 @@ func (it *bigEndianPostings) Seek(x storage.SeriesRef) bool {
 		return true
 	}
 
-	l := it.list[it.i:]
+	start := it.i
+	if start < 0 {
+		start = 0
+	}
+
+	l := it.list[start:]
 	num := len(l) / 4
 	// Do binary search between current position and end.
 	i := sort.Search(num, func(i int) bool {
@@ -930,7 +940,7 @@ func (it *bigEndianPostings) Seek(x storage.SeriesRef) bool {
 	if i < num {
 		j := i * 4
 		it.cur = binary.BigEndian.Uint32(l[j:])
-		it.i += j
+		it.i = start + j
 		return true
 	}