diff --git a/weed/server/filer_server_handlers_write_upload.go b/weed/server/filer_server_handlers_write_upload.go
index 32a722507..e28309c6a 100644
--- a/weed/server/filer_server_handlers_write_upload.go
+++ b/weed/server/filer_server_handlers_write_upload.go
@@ -29,7 +29,6 @@ var bufPool = sync.Pool{
 }
 
 func (fs *FilerServer) uploadReaderToChunks(w http.ResponseWriter, r *http.Request, reader io.Reader, chunkSize int32, fileName, contentType string, contentLength int64, so *operation.StorageOption) ([]*filer_pb.FileChunk, hash.Hash, int64, error, []byte) {
-	var fileChunks []*filer_pb.FileChunk
 
 	md5Hash := md5.New()
 	var partReader = ioutil.NopCloser(io.TeeReader(reader, md5Hash))
@@ -41,6 +40,8 @@ func (fs *FilerServer) uploadReaderToChunks(w http.ResponseWriter, r *http.Reque
 	var wg sync.WaitGroup
 	var bytesBufferCounter int64
 	bytesBufferLimitCond := sync.NewCond(new(sync.Mutex))
+	var fileChunks []*filer_pb.FileChunk
+	var fileChunksLock sync.Mutex
 	for {
 
 		// need to throttle used byte buffer
@@ -94,7 +95,9 @@ func (fs *FilerServer) uploadReaderToChunks(w http.ResponseWriter, r *http.Reque
 				uploadErr = toChunkErr
 			}
 			if chunk != nil {
+				fileChunksLock.Lock()
 				fileChunks = append(fileChunks, chunk)
+				fileChunksLock.Unlock()
 				glog.V(4).Infof("uploaded %s chunk %d to %s [%d,%d)", fileName, len(fileChunks), chunk.FileId, offset, offset+int64(chunk.Size))
 			}
 		}(chunkOffset)