diff --git a/tools/assetmgr/assetmgr.py b/tools/assetmgr/assetmgr.py
index 81ecf4230..e518e10bc 100644
--- a/tools/assetmgr/assetmgr.py
+++ b/tools/assetmgr/assetmgr.py
@@ -160,7 +160,7 @@ def writefile(filename, contents):
     # check if it's in args
     for arg in sys.argv:
         if arg.startswith('--romid='):
-            os.environ['ROMID'] = arg.removeprefix('--romid=')
+            os.environ['ROMID'] = arg[len('--romid='):]
             break
     else:
         # default to ntsc-final