diff --git a/builder/main.py b/builder/main.py
index dd384590..a76b0823 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -273,7 +273,7 @@ def __configure_upload_port(env):
             '"%s"' % (platform.get_package_dir("tool-stm32duino") or ""),
             "stm32flash", "stm32flash"),
         UPLOADERFLAGS=[
-            "-i %s" % board.get("upload.gpiostring") if board.get("upload.gpiostring", False) else "",
+            "-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"
         ],