diff --git a/util/segmentmap/segmentmap.go b/util/segmentmap/segmentmap.go
index 1dc9f24f3d..7f1e31fdf9 100644
--- a/util/segmentmap/segmentmap.go
+++ b/util/segmentmap/segmentmap.go
@@ -20,6 +20,7 @@ import (
 )
 
 // SegmentMap is used for handle a big map slice by slice
+// It's not thread safe
 type SegmentMap struct {
 	size int
 	maps []map[string]interface{}
@@ -48,17 +49,17 @@ func (sm *SegmentMap) Get(key []byte) (interface{}, bool) {
 // GetSegment gets the map specific by index
 func (sm *SegmentMap) GetSegment(index int) (map[string]interface{}, error) {
 	if index >= len(sm.maps) {
-		return nil, errors.Errorf("index out of bound")
+		return nil, errors.Errorf("index out of bound: %d", index)
 	}
 
 	return sm.maps[index], nil
 }
 
 // Set if empty, return whether already exists
-func (sm *SegmentMap) Set(key []byte, value interface{}, force bool) (exist bool) {
+func (sm *SegmentMap) Set(key []byte, value interface{}, force bool) bool {
 	idx := int(crc32.ChecksumIEEE(key)) % sm.size
 	k := string(key)
-	_, exist = sm.maps[idx][k]
+	_, exist := sm.maps[idx][k]
 	if exist && !force {
 		return exist
 	}
diff --git a/util/segmentmap/segmentmap_test.go b/util/segmentmap/segmentmap_test.go
index 32c350d3fd..b8610a5d90 100644
--- a/util/segmentmap/segmentmap_test.go
+++ b/util/segmentmap/segmentmap_test.go
@@ -29,14 +29,14 @@ func (s *testSegmentMapSuite) TestSegment(c *C) {
 	k := []byte("k")
 	v := []byte("v")
 	val, exist := m.Get(k)
-	c.Assert(exist, Equals, false)
+	c.Assert(exist, IsFalse)
 
 	exist = m.Set(k, v, false)
 	c.Assert(exist, IsFalse)
 
 	val, exist = m.Get(k)
 	c.Assert(v, DeepEquals, val.([]byte))
-	c.Assert(exist, Equals, true)
+	c.Assert(exist, IsTrue)
 
 	m0, err := m.GetSegment(0)
 	c.Assert(err, IsNil)