diff --git a/config/default.nix b/config/default.nix
index 5224ee42..11311286 100644
--- a/config/default.nix
+++ b/config/default.nix
@@ -4,6 +4,13 @@
 # options should go to modules.
 
 {
+  assertions = [
+    {
+      assertion = config.system.replaceRuntimeDependencies == [];
+      message = "system.replaceRuntimeDependencies causes hydra to build the system at evaluation time. It must be removed!";
+    }
+  ];
+
   boot = {
     cleanTmpDir = true;
     kernel.sysctl = {