diff --git a/src/Arduino_ESP32_OTA.cpp b/src/Arduino_ESP32_OTA.cpp
index 9839e0a..ffb398c 100644
--- a/src/Arduino_ESP32_OTA.cpp
+++ b/src/Arduino_ESP32_OTA.cpp
@@ -44,6 +44,7 @@ static void write_byte(uint8_t data) {
   if(_esp_ota_obj_ptr) {
     _esp_ota_obj_ptr->write_byte_to_flash(data);
   }
+  return -1;
 }
 
 /******************************************************************************