diff --git a/builder/main.py b/builder/main.py
index 1e3ea14..d3afded 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -278,7 +278,7 @@ if upload_protocol == "esptool":
         UPLOADOTACMD='"$PYTHONEXE" "$UPLOADEROTA" $UPLOADEROTAFLAGS -f $SOURCE',
     )
     for image in env.get("FLASH_EXTRA_IMAGES", []):
-        env.Append(UPLOADERFLAGS=[image[0], image[1]])
+        env.Append(UPLOADERFLAGS=[image[0], env.subst(image[1])])
 
     if "uploadfs" in COMMAND_LINE_TARGETS:
         env.Replace(