diff --git a/src/allmydata/test/common.py b/src/allmydata/test/common.py
index a420dd3ba..34fca6481 100644
--- a/src/allmydata/test/common.py
+++ b/src/allmydata/test/common.py
@@ -221,7 +221,7 @@ class UseNode(object):
     """
     plugin_config = attr.ib()
     storage_plugin = attr.ib()
-    basedir = attr.ib()
+    basedir = attr.ib(validator=attr.validators.instance_of(FilePath))
     introducer_furl = attr.ib()
     node_config = attr.ib(default=attr.Factory(dict))