diff --git a/buildbot_nix/__init__.py b/buildbot_nix/__init__.py
index 9eef54a..9e89d7f 100644
--- a/buildbot_nix/__init__.py
+++ b/buildbot_nix/__init__.py
@@ -5,7 +5,7 @@ import sys
 import graphlib
 import base64
 from collections.abc import Generator
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
@@ -47,9 +47,9 @@ class NixBuilder:
     publicHostKey: str | None = None
     sshUser: str | None = None
     sshKey: str | None = None
-    systems: list[str] = ["-"]
-    supportedFeatures: list[str] = ["-"]
-    mandatoryFeatures: list[str] = ["-"]
+    systems: list[str] = field(default_factory=lambda: ["-"])
+    supportedFeatures: list[str] = field(default_factory=lambda: ["-"])
+    mandatoryFeatures: list[str] = field(default_factory=lambda: ["-"])
 
     def to_nix_line(self):
         encoded_public_key = base64.b64encode(self.publicHostKey.encode('ascii')).decode('ascii') if self.publicHostKey is not None else "-"