diff --git a/src/envelope_crypto/envelope_crypto.py b/src/envelope_crypto/envelope_crypto.py
index ea76224..521c69f 100644
--- a/src/envelope_crypto/envelope_crypto.py
+++ b/src/envelope_crypto/envelope_crypto.py
@@ -681,7 +681,9 @@ def _fingerprint_normalize(value: Any) -> Any:
     if isinstance(value, (set, frozenset)):
         members = [_fingerprint_normalize(item) for item in value]
         members.sort(key=lambda item: json.dumps(item, sort_keys=True, default=_fingerprint_default))
-        return {"\x00set:members": members}
+        # keep the concrete type in the marker so a set value and a frozenset value with the
+        # same members stay distinct; the \x00 prefix keeps it uncollidable with a user str key
+        return {f"\x00{type(value).__name__}:members": members}
     if isinstance(value, (list, tuple)):
         return [_fingerprint_normalize(item) for item in value]
     return value