diff --git a/buildroot/share/PlatformIO/scripts/marlin.py b/buildroot/share/PlatformIO/scripts/marlin.py
index 169dd9d3c3..8d8dbb5b63 100644
--- a/buildroot/share/PlatformIO/scripts/marlin.py
+++ b/buildroot/share/PlatformIO/scripts/marlin.py
@@ -16,7 +16,8 @@ def copytree(src, dst, symlinks=False, ignore=None):
             shutil.copy2(item, dst / item.name)
 
 def replace_define(field, value):
-    for define in env['CPPDEFINES']:
+    envdefs = env['CPPDEFINES'].copy()
+    for define in envdefs:
         if define[0] == field:
             env['CPPDEFINES'].remove(define)
     env['CPPDEFINES'].append((field, value))