diff --git a/tsdb/series_segment.go b/tsdb/series_segment.go
index 0fab853f8f..b91ebe018c 100644
--- a/tsdb/series_segment.go
+++ b/tsdb/series_segment.go
@@ -118,7 +118,7 @@ func (s *SeriesSegment) InitForWrite() (err error) {
 	// Only calculcate segment data size if writing.
 	for s.size = uint32(SeriesSegmentHeaderSize); s.size < uint32(len(s.data)); {
 		flag, _, _, sz := ReadSeriesEntry(s.data[s.size:])
-		if flag == 0 {
+		if !IsValidSeriesEntryFlag(flag) {
 			break
 		}
 		s.size += uint32(sz)
@@ -237,7 +237,7 @@ func (s *SeriesSegment) MaxSeriesID() uint64 {
 func (s *SeriesSegment) ForEachEntry(fn func(flag uint8, id uint64, offset int64, key []byte) error) error {
 	for pos := uint32(SeriesSegmentHeaderSize); pos < uint32(len(s.data)); {
 		flag, id, key, sz := ReadSeriesEntry(s.data[pos:])
-		if flag == 0 {
+		if !IsValidSeriesEntryFlag(flag) {
 			break
 		}
 
@@ -368,7 +368,7 @@ func (hdr *SeriesSegmentHeader) WriteTo(w io.Writer) (n int64, err error) {
 func ReadSeriesEntry(data []byte) (flag uint8, id uint64, key []byte, sz int64) {
 	// If flag byte is zero then no more entries exist.
 	flag, data = uint8(data[0]), data[1:]
-	if flag == 0 {
+	if !IsValidSeriesEntryFlag(flag) {
 		return 0, 0, nil, 1
 	}
 
@@ -396,3 +396,13 @@ func AppendSeriesEntry(dst []byte, flag uint8, id uint64, key []byte) []byte {
 	}
 	return dst
 }
+
+// IsValidSeriesEntryFlag returns true if flag is valid.
+func IsValidSeriesEntryFlag(flag byte) bool {
+	switch flag {
+	case SeriesEntryInsertFlag, SeriesEntryTombstoneFlag:
+		return true
+	default:
+		return false
+	}
+}
diff --git a/tsdb/series_segment_test.go b/tsdb/series_segment_test.go
index fe4f87c8fd..a98eab06dd 100644
--- a/tsdb/series_segment_test.go
+++ b/tsdb/series_segment_test.go
@@ -2,6 +2,7 @@ package tsdb_test
 
 import (
 	"bytes"
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -140,6 +141,49 @@ func TestSeriesSegmentHeader(t *testing.T) {
 	}
 }
 
+func TestSeriesSegment_PartialWrite(t *testing.T) {
+	dir, cleanup := MustTempDir()
+	defer cleanup()
+
+	// Create a new initial segment (4mb) and initialize for writing.
+	segment, err := tsdb.CreateSeriesSegment(0, filepath.Join(dir, "0000"))
+	if err != nil {
+		t.Fatal(err)
+	} else if err := segment.InitForWrite(); err != nil {
+		t.Fatal(err)
+	}
+	defer segment.Close()
+
+	// Write two entries.
+	if _, err := segment.WriteLogEntry(tsdb.AppendSeriesEntry(nil, tsdb.SeriesEntryInsertFlag, 1, tsdb.AppendSeriesKey(nil, []byte("A"), nil))); err != nil {
+		t.Fatal(err)
+	} else if _, err := segment.WriteLogEntry(tsdb.AppendSeriesEntry(nil, tsdb.SeriesEntryInsertFlag, 2, tsdb.AppendSeriesKey(nil, []byte("B"), nil))); err != nil {
+		t.Fatal(err)
+	}
+	sz := segment.Size()
+	entrySize := len(tsdb.AppendSeriesEntry(nil, tsdb.SeriesEntryInsertFlag, 2, tsdb.AppendSeriesKey(nil, []byte("B"), nil)))
+
+	// Close segment.
+	if err := segment.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Truncate at each point and reopen.
+	for i := entrySize; i > 0; i-- {
+		if err := os.Truncate(filepath.Join(dir, "0000"), sz-int64(entrySize-i)); err != nil {
+			t.Fatal(err)
+		}
+		segment := tsdb.NewSeriesSegment(0, filepath.Join(dir, "0000"))
+		if err := segment.Open(); err != nil {
+			t.Fatal(err)
+		} else if err := segment.InitForWrite(); err != nil {
+			t.Fatal(err)
+		} else if err := segment.Close(); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
 func TestJoinSeriesOffset(t *testing.T) {
 	if offset := tsdb.JoinSeriesOffset(0x1234, 0x56789ABC); offset != 0x123456789ABC {
 		t.Fatalf("unexpected offset: %x", offset)