diff --git a/buildroot/share/PlatformIO/scripts/marlin.py b/buildroot/share/PlatformIO/scripts/marlin.py
index d83ebceee244..d534c5bb0cf4 100644
--- a/buildroot/share/PlatformIO/scripts/marlin.py
+++ b/buildroot/share/PlatformIO/scripts/marlin.py
@@ -17,7 +17,7 @@ def copytree(src, dst, symlinks=False, ignore=None):
             shutil.copy2(s, d)
 
 def replace_define(field, value):
-	for define in env['CPPDEFINES']:
+	for define in list(env['CPPDEFINES']):
 		if define[0] == field:
 			env['CPPDEFINES'].remove(define)
 	env['CPPDEFINES'].append((field, value))