def __init__(
     self,
     credential_id: bytes,
     is_resident_credential: bool,
     rp_id: Optional[str],
     user_handle: Optional[bytes],
     private_key: bytes,
     sign_count: int,
 ):
     """Constructor. A credential stored in a virtual authenticator.
     https://w3c.github.io/webauthn/#credential-parameters.
 
     :Args:
         - credential_id (bytes): Unique base64 encoded string.
         - is_resident_credential (bool): Whether the credential is client-side discoverable.
-        - rp_id (str): Relying party identifier.
+        - rp_id (str | None): Relying party identifier. Optional for resident credentials.
         - user_handle (bytes): userHandle associated to the credential. Must be Base64 encoded string. Can be None.
         - private_key (bytes): Base64 encoded PKCS#8 private key.
         - sign_count (int): initial value for a signature counter.
     """
     self._id = credential_id
     self._is_resident_credential = is_resident_credential
-    self._rp_id = rp_id
+    # Default to empty string if rp_id is None to keep property type stable
+    # Alternatively, enforce presence for non-resident credentials
+    if not is_resident_credential and rp_id is None:
+        raise ValueError("rp_id is required for non-resident credentials")
+    self._rp_id = rp_id or ""
     self._user_handle = user_handle
     self._private_key = private_key
     self._sign_count = sign_count

 @classmethod
 def from_dict(cls, data: dict[str, Any]) -> "Credential":
     _id = urlsafe_b64decode(f"{data['credentialId']}==")
     is_resident_credential = bool(data["isResidentCredential"])
-    rp_id = data.get("rpId", None)
+    rp_id_val = data.get("rpId") or ""
     private_key = urlsafe_b64decode(f"{data['privateKey']}==")
     sign_count = int(data["signCount"])
-    user_handle = urlsafe_b64decode(f"{data['userHandle']}==") if data.get("userHandle", None) else None
+    user_handle = urlsafe_b64decode(f"{data['userHandle']}==") if data.get("userHandle") else None
 
-    return cls(_id, is_resident_credential, rp_id, user_handle, private_key, sign_count)
+    return cls(_id, is_resident_credential, rp_id_val, user_handle, private_key, sign_count)

 def __init__(
         self,
         credential_id: bytes,
         is_resident_credential: bool,
         rp_id: Optional[str],
         user_handle: Optional[bytes],
         private_key: bytes,
         sign_count: int,
     ):
         """Constructor. A credential stored in a virtual authenticator.
         https://w3c.github.io/webauthn/#credential-parameters.
 
         :Args:
             - credential_id (bytes): Unique base64 encoded string.
             - is_resident_credential (bool): Whether the credential is client-side discoverable.
-            - rp_id (str): Relying party identifier.
-            - user_handle (bytes): userHandle associated to the credential. Must be Base64 encoded string. Can be None.
+            - rp_id (Optional[str]): Relying party identifier. May be None in scenarios where RP ID is not applicable.
+            - user_handle (Optional[bytes]): userHandle associated to the credential. Must be Base64 encoded string. Can be None.
             - private_key (bytes): Base64 encoded PKCS#8 private key.
-            - sign_count (int): initial value for a signature counter.
+            - sign_count (int): Initial value for a signature counter.
         """

 def __str__(self) -> str:
-    return f"Credential(id={self.id}, is_resident_credential={self.is_resident_credential}, rp_id={self.rp_id},\
-        user_handle={self.user_handle}, private_key={self.private_key}, sign_count={self.sign_count})"
+    rp = self.rp_id if self.rp_id is not None else ""
+    return (
+        f"Credential(id={self.id}, is_resident_credential={self.is_resident_credential}, "
+        f"rp_id={rp}, user_handle={self.user_handle}, private_key={self.private_key}, "
+        f"sign_count={self.sign_count})"
+    )