diff --git a/go/operation/assign_file_id.go b/go/operation/assign_file_id.go
index 4941d3c86..018e1d763 100644
--- a/go/operation/assign_file_id.go
+++ b/go/operation/assign_file_id.go
@@ -10,11 +10,11 @@ import (
 )
 
 type AssignResult struct {
-	Fid       string `json:"fid"`
-	Url       string `json:"url"`
-	PublicUrl string `json:"publicUrl"`
-	Count     int
-	Error     string `json:"error"`
+	Fid       string `json:"fid,omitempty"`
+	Url       string `json:"url,omitempty"`
+	PublicUrl string `json:"publicUrl,omitempty"`
+	Count     int    `json:"count,omitempty"`
+	Error     string `json:"error,omitempty"`
 }
 
 func Assign(server string, count int, replication string, collection string) (*AssignResult, error) {
diff --git a/go/weed/weed_server/master_server_handlers.go b/go/weed/weed_server/master_server_handlers.go
index 32422d497..cc83a748f 100644
--- a/go/weed/weed_server/master_server_handlers.go
+++ b/go/weed/weed_server/master_server_handlers.go
@@ -74,21 +74,21 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 	option, err := ms.getVolumeGrowOption(r)
 	if err != nil {
 		w.WriteHeader(http.StatusNotAcceptable)
-		writeJsonQuiet(w, r, map[string]string{"error": err.Error()})
+		writeJsonQuiet(w, r, AssignResult{Error: err.Error()})
 		return
 	}
 
 	if !ms.Topo.HasWriableVolume(option) {
 		if ms.Topo.FreeSpace() <= 0 {
 			w.WriteHeader(http.StatusNotFound)
-			writeJsonQuiet(w, r, map[string]string{"error": "No free volumes left!"})
+			writeJsonQuiet(w, r, AssignResult{Error: "No free volumes left!"})
 			return
 		} else {
 			ms.vgLock.Lock()
 			defer ms.vgLock.Unlock()
 			if !ms.Topo.HasWriableVolume(option) {
 				if _, err = ms.vg.AutomaticGrowByType(option, ms.Topo); err != nil {
-					writeJsonQuiet(w, r, map[string]string{"error": "Cannot grow volume group! " + err.Error()})
+					writeJsonQuiet(w, r, AssignResult{Error: "Cannot grow volume group! " + err.Error()})
 					return
 				}
 			}
@@ -96,9 +96,9 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 	}
 	fid, count, dn, err := ms.Topo.PickForWrite(requestedCount, option)
 	if err == nil {
-		writeJsonQuiet(w, r, map[string]interface{}{"fid": fid, "url": dn.Url(), "publicUrl": dn.PublicUrl, "count": count})
+		writeJsonQuiet(w, r, AssignResult{Fid: fid, Url: dn.Url(), PublicUrl: dn.PublicUrl, Count: count})
 	} else {
 		w.WriteHeader(http.StatusNotAcceptable)
-		writeJsonQuiet(w, r, map[string]string{"error": err.Error()})
+		writeJsonQuiet(w, r, AssignResult{Error: err.Error()})
 	}
 }