diff --git a/buildbot_nix/__init__.py b/buildbot_nix/__init__.py
index 1d49d7e..97e0016 100644
--- a/buildbot_nix/__init__.py
+++ b/buildbot_nix/__init__.py
@@ -531,6 +531,14 @@ def nix_eval_config(
     For each evaluated attribute a new build pipeline is started.
     """
     factory = util.BuildFactory()
+
+    gerrit_private_key = None
+    with open(project.private_sshkey_path, 'r') as f:
+        gerrit_private_key = f.read()
+
+    if gerrit_private_key is None:
+        raise RuntimeError('No gerrit private key to fetch the repositories')
+
     # check out the source
     factory.addStep(
         steps.Gerrit(
@@ -538,9 +546,10 @@ def nix_eval_config(
             mode="full",
             retry=[60, 60],
             timeout=3600,
-            sshPrivateKey=project.private_sshkey_path
+            sshPrivateKey=gerrit_private_key
         ),
     )
+
     # use one gcroots directory per worker. this should be scoped to the largest unique resource
     # in charge of builds (ie, buildnumber is too narrow) to not litter the system with permanent
     # gcroots in case of worker restarts.
@@ -751,12 +760,6 @@ def config_for_project(
             ),
         ],
     )
-    gerrit_private_key = None
-    with open(project.private_sshkey_path, 'r') as f:
-        gerrit_private_key = f.read()
-
-    if gerrit_private_key is None:
-        raise RuntimeError('No gerrit private key to fetch the repositories')
 
     config["builders"].extend(
         [