diff --git a/artiq/frontend/artiq_flash.py b/artiq/frontend/artiq_flash.py
index 12475cf0b..532d1da92 100755
--- a/artiq/frontend/artiq_flash.py
+++ b/artiq/frontend/artiq_flash.py
@@ -347,7 +347,10 @@ def main():
         raise ValueError("the directory containing the binaries need to be specified using -d.")
 
     binary_dir = args.dir
-    rtm_binary_dir = os.path.join(binary_dir, "rtm")
+    if binary_dir is not None:
+        rtm_binary_dir = os.path.join(binary_dir, "rtm")
+    else:
+        rtm_binary_dir = None
 
     if args.host is None:
         client = LocalClient()