diff --git a/wagtail/wagtailcore/whitelist.py b/wagtail/wagtailcore/whitelist.py
index 0859ccfa8b..4aaff780d8 100644
--- a/wagtail/wagtailcore/whitelist.py
+++ b/wagtail/wagtailcore/whitelist.py
@@ -29,7 +29,7 @@ def attribute_rule(allowed_attrs):
     * if the lookup returns a truthy value, keep the attribute; if falsy, drop it
     """
     def fn(tag):
-        for attr, val in tag.attrs.items():
+        for attr, val in list(tag.attrs.items()):
             rule = allowed_attrs.get(attr)
             if rule:
                 if callable(rule):