diff --git a/src/index.ts b/src/index.ts
index 7b97065..e5e307f 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -209,13 +209,14 @@ export default class Updater {
 
         private async sendNextPacket(): Promise<boolean> {
                 let packet = this.file.slice(this._fileProgress, this._fileProgress+this._packetSize);
+                this._fileProgress += this._packetSize;
                 return new Promise((resolve, reject) => {
                         this.bleObject.write(this.bleDeviceId, this._updaterServiceUUID, this._updateFileCharacteristicUUID, packet.buffer,
                                 () => {
-                                        this._fileProgress += this._packetSize;
                                         resolve(true);
                                 },
                                 (error) => {
+                                        this._fileProgress -= this._packetSize;
                                         reject(error);
                                 }
                         )