diff --git a/fileserver/commitmgr/commitmgr.go b/fileserver/commitmgr/commitmgr.go
index 5ab5e31..f719e71 100644
--- a/fileserver/commitmgr/commitmgr.go
+++ b/fileserver/commitmgr/commitmgr.go
@@ -22,8 +22,8 @@ type Commit struct {
 	CreatorID      string `json:"creator"`
 	Desc           string `json:"description"`
 	Ctime          int64  `json:"ctime"`
-	ParentID       string `json:"parent_id,omitempty"`
-	SecondParentID string `json:"second_parent_id,omitempty"`
+	ParentID       String `json:"parent_id"`
+	SecondParentID String `json:"second_parent_id"`
 	RepoName       string `json:"repo_name"`
 	RepoDesc       string `json:"repo_desc"`
 	RepoCategory   string `json:"repo_category"`
@@ -57,7 +57,7 @@ func NewCommit(repoID, parentID, newRoot, user, desc string) *Commit {
 	commit.CreatorID = "0000000000000000000000000000000000000000"
 	commit.Ctime = time.Now().Unix()
 	commit.CommitID = computeCommitID(commit)
-	commit.ParentID = parentID
+	commit.ParentID.SetValid(parentID)
 
 	return commit
 }
diff --git a/fileserver/commitmgr/commitmgr_test.go b/fileserver/commitmgr/commitmgr_test.go
index fd4501d..a08d928 100644
--- a/fileserver/commitmgr/commitmgr_test.go
+++ b/fileserver/commitmgr/commitmgr_test.go
@@ -48,7 +48,7 @@ func TestCommit(t *testing.T) {
 	newCommit.CreatorID = commitID
 	newCommit.Desc = "This is a commit"
 	newCommit.Ctime = time.Now().Unix()
-	newCommit.ParentID = commitID
+	newCommit.ParentID.SetValid(commitID)
 	newCommit.DeviceName = "Linux"
 	err := Save(newCommit)
 	if err != nil {
diff --git a/fileserver/commitmgr/null.go b/fileserver/commitmgr/null.go
new file mode 100644
index 0000000..f085aea
--- /dev/null
+++ b/fileserver/commitmgr/null.go
@@ -0,0 +1,114 @@
+package commitmgr
+
+import (
+	"bytes"
+	"database/sql"
+	"encoding/json"
+	"fmt"
+)
+
+// nullBytes is a JSON null literal
+var nullBytes = []byte("null")
+
+// String is a nullable string. It supports SQL and JSON serialization.
+// It will marshal to null if null. Blank string input will be considered null.
+type String struct {
+	sql.NullString
+}
+
+// StringFrom creates a new String that will never be blank.
+func StringFrom(s string) String {
+	return NewString(s, true)
+}
+
+// StringFromPtr creates a new String that be null if s is nil.
+func StringFromPtr(s *string) String {
+	if s == nil {
+		return NewString("", false)
+	}
+	return NewString(*s, true)
+}
+
+// ValueOrZero returns the inner value if valid, otherwise zero.
+func (s String) ValueOrZero() string {
+	if !s.Valid {
+		return ""
+	}
+	return s.String
+}
+
+// NewString creates a new String
+func NewString(s string, valid bool) String {
+	return String{
+		NullString: sql.NullString{
+			String: s,
+			Valid:  valid,
+		},
+	}
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+// It supports string and null input. Blank string input does not produce a null String.
+func (s *String) UnmarshalJSON(data []byte) error {
+	if bytes.Equal(data, nullBytes) {
+		s.Valid = false
+		return nil
+	}
+
+	if err := json.Unmarshal(data, &s.String); err != nil {
+		return fmt.Errorf("null: couldn't unmarshal JSON: %w", err)
+	}
+
+	s.Valid = true
+	return nil
+}
+
+// MarshalJSON implements json.Marshaler.
+// It will encode null if this String is null.
+func (s String) MarshalJSON() ([]byte, error) {
+	if !s.Valid {
+		return []byte("null"), nil
+	}
+	return json.Marshal(s.String)
+}
+
+// MarshalText implements encoding.TextMarshaler.
+// It will encode a blank string when this String is null.
+func (s String) MarshalText() ([]byte, error) {
+	if !s.Valid {
+		return []byte{}, nil
+	}
+	return []byte(s.String), nil
+}
+
+// UnmarshalText implements encoding.TextUnmarshaler.
+// It will unmarshal to a null String if the input is a blank string.
+func (s *String) UnmarshalText(text []byte) error {
+	s.String = string(text)
+	s.Valid = s.String != ""
+	return nil
+}
+
+// SetValid changes this String's value and also sets it to be non-null.
+func (s *String) SetValid(v string) {
+	s.String = v
+	s.Valid = true
+}
+
+// Ptr returns a pointer to this String's value, or a nil pointer if this String is null.
+func (s String) Ptr() *string {
+	if !s.Valid {
+		return nil
+	}
+	return &s.String
+}
+
+// IsZero returns true for null strings, for potential future omitempty support.
+func (s String) IsZero() bool {
+	return !s.Valid
+}
+
+// Equal returns true if both strings have the same value or are both null.
+func (s String) Equal(other String) bool {
+	return s.Valid == other.Valid && (!s.Valid || s.String == other.String)
+}
diff --git a/fileserver/fileop.go b/fileserver/fileop.go
index 8083a18..8ec0c55 100644
--- a/fileserver/fileop.go
+++ b/fileserver/fileop.go
@@ -1671,7 +1671,7 @@ func genCommitNeedRetry(repo *repomgr.Repo, base *commitmgr.Commit, commit *comm
 
 		mergedCommit = commitmgr.NewCommit(repoID, currentHead.CommitID, opt.mergedRoot, user, mergeDesc)
 		repomgr.RepoToCommit(repo, mergedCommit)
-		mergedCommit.SecondParentID = commit.CommitID
+		mergedCommit.SecondParentID.SetValid(commit.CommitID)
 		mergedCommit.NewMerge = 1
 		if opt.conflict {
 			mergedCommit.Conflict = 1
diff --git a/fileserver/fsmgr/fsmgr.go b/fileserver/fsmgr/fsmgr.go
index bb336bc..bee775b 100644
--- a/fileserver/fsmgr/fsmgr.go
+++ b/fileserver/fsmgr/fsmgr.go
@@ -19,8 +19,8 @@ import (
 // Seafile is a file object
 type Seafile struct {
 	Version  int      `json:"version"`
-	FileType int      `json:"type,omitempty"`
-	FileID   string   `json:"file_id,omitempty"`
+	FileType int      `json:"type"`
+	FileID   string   `json:"file_id"`
 	FileSize uint64   `json:"size"`
 	BlkIDs   []string `json:"block_ids"`
 }
@@ -38,8 +38,8 @@ type SeafDirent struct {
 //SeafDir is a dir object
 type SeafDir struct {
 	Version int           `json:"version"`
-	DirType int           `json:"type,omitempty"`
-	DirID   string        `json:"dir_id,omitempty"`
+	DirType int           `json:"type"`
+	DirID   string        `json:"dir_id"`
 	Entries []*SeafDirent `json:"dirents"`
 }
 
diff --git a/fileserver/sync_api.go b/fileserver/sync_api.go
index 97ff8ed..6da5f0b 100644
--- a/fileserver/sync_api.go
+++ b/fileserver/sync_api.go
@@ -909,9 +909,9 @@ func putUpdateBranchCB(rsp http.ResponseWriter, r *http.Request) *appError {
 		return &appError{err, "", http.StatusInternalServerError}
 	}
 
-	base, err := commitmgr.Load(repoID, newCommit.ParentID)
+	base, err := commitmgr.Load(repoID, newCommit.ParentID.String)
 	if err != nil {
-		err := fmt.Errorf("Failed to get commit %s for repo %s", newCommit.ParentID, repoID)
+		err := fmt.Errorf("Failed to get commit %s for repo %s", newCommit.ParentID.String, repoID)
 		return &appError{err, "", http.StatusInternalServerError}
 	}
 
diff --git a/fileserver/virtual_repo.go b/fileserver/virtual_repo.go
index 4a7c4c2..be4c044 100644
--- a/fileserver/virtual_repo.go
+++ b/fileserver/virtual_repo.go
@@ -191,9 +191,9 @@ func cleanupVirtualRepos(repoID string) error {
 }
 
 func handleMissingVirtualRepo(repo *repomgr.Repo, head *commitmgr.Commit, vInfo *repomgr.VRepoInfo) (string, error) {
-	parent, err := commitmgr.Load(head.RepoID, head.ParentID)
+	parent, err := commitmgr.Load(head.RepoID, head.ParentID.String)
 	if err != nil {
-		err := fmt.Errorf("failed to load commit %s/%s : %v", head.RepoID, head.ParentID, err)
+		err := fmt.Errorf("failed to load commit %s/%s : %v", head.RepoID, head.ParentID.String, err)
 		return "", err
 	}