diff --git a/csp/contrib/formatted_policy.py b/csp/contrib/formatted_policy.py
index 60fbb73..6592aef 100644
--- a/csp/contrib/formatted_policy.py
+++ b/csp/contrib/formatted_policy.py
@@ -24,7 +24,10 @@ def build_policy(self, request, response):
             request, response,
         ):
             format_kwargs = {
-                field_name for _, field_name, _, _ in formatter.parse(csp)
+                # field_name ::= arg_name ("." attribute_name | "[" element_index "]")*
+                field_name[: next((i for i, c in enumerate(field_name) if c in ".["), None)]
+                for _, field_name, _, _ in formatter.parse(csp)
+                if field_name is not None
             }
             if format_kwargs:
                 for name in format_kwargs: