diff --git a/builder/main.py b/builder/main.py
index 9125e69d..dda37b6d 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -267,13 +267,13 @@ def __configure_upload_port(env):
 elif upload_protocol == "serial":
     def __configure_upload_port(env):
         return env.subst("$UPLOAD_PORT")
-
     env.Replace(
         __configure_upload_port=__configure_upload_port,
         UPLOADER=join(
             '"%s"' % platform.get_package_dir("tool-stm32duino") or "",
             "stm32flash", "stm32flash"),
         UPLOADERFLAGS=[
+            "-i%s" % board.get("upload.gpiostring") if board.get("upload.gpiostring", False) else "",
             "-g", board.get("upload.offset_address", "0x08000000"),
             "-b", env.subst("$UPLOAD_SPEED") or "115200", "-w"
         ],