diff --git a/tsdb/index/index.go b/tsdb/index/index.go
index 957e468f6..0d2981385 100644
--- a/tsdb/index/index.go
+++ b/tsdb/index/index.go
@@ -18,6 +18,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/binary"
+	"fmt"
 	"hash"
 	"hash/crc32"
 	"io"
@@ -1782,7 +1783,13 @@ func (dec *Decoder) Postings(b []byte) (int, Postings, error) {
 	d := encoding.Decbuf{B: b}
 	n := d.Be32int()
 	l := d.Get()
-	return n, newBigEndianPostings(l), d.Err()
+	if d.Err() != nil {
+		return 0, nil, d.Err()
+	}
+	if len(l) != 4*n {
+		return 0, nil, fmt.Errorf("unexpected postings length, should be %d bytes for %d postings, got %d bytes", 4*n, n, len(l))
+	}
+	return n, newBigEndianPostings(l), nil
 }
 
 // LabelNamesOffsetsFor decodes the offsets of the name symbols for a given series.
diff --git a/tsdb/index/index_test.go b/tsdb/index/index_test.go
index c859dbb47..e2764ff26 100644
--- a/tsdb/index/index_test.go
+++ b/tsdb/index/index_test.go
@@ -560,3 +560,8 @@ func TestSymbols(t *testing.T) {
 	}
 	require.NoError(t, iter.Err())
 }
+
+func TestDecoder_Postings_WrongInput(t *testing.T) {
+	_, _, err := (&Decoder{}).Postings([]byte("the cake is a lie"))
+	require.Error(t, err)
+}