diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index 254e9266e89e..848587bde294 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/nixos/modules/system/activation/top-level.nix
@@ -93,19 +93,12 @@ let
       ${config.system.extraSystemBuilderCmds}
     '';
 
-  # Handle assertions
-
-  failed = map (x: x.message) (filter (x: !x.assertion) config.assertions);
-
-  showWarnings = res: fold (w: x: builtins.trace "warning: ${w}" x) res config.warnings;
-
   # Putting it all together.  This builds a store path containing
   # symlinks to the various parts of the built configuration (the
   # kernel, systemd units, init scripts, etc.) as well as a script
   # `switch-to-configuration' that activates the configuration and
   # makes it bootable.
-  baseSystem = showWarnings (
-    if [] == failed then pkgs.stdenvNoCC.mkDerivation {
+  baseSystem = pkgs.stdenvNoCC.mkDerivation {
       name = let hn = config.networking.hostName;
                  nn = if (hn != "") then hn else "unnamed";
           in "nixos-system-${nn}-${config.system.nixos.label}";
@@ -130,12 +123,22 @@ let
       # Needed by switch-to-configuration.
 
       perl = "${pkgs.perl}/bin/perl " + (concatMapStringsSep " " (lib: "-I${lib}/${pkgs.perl.libPrefix}") (with pkgs.perlPackages; [ FileSlurp NetDBus XMLParser XMLTwig ]));
-  } else throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}");
+  };
+
+  # Handle assertions and warnings
+
+  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+
+  showWarnings = res: fold (w: x: builtins.trace "warning: ${w}" x) res config.warnings;
+
+  baseSystemAssertWarn = if failedAssertions != []
+    then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
+    else showWarnings baseSystem;
 
   # Replace runtime dependencies
   system = fold ({ oldDependency, newDependency }: drv:
       pkgs.replaceDependency { inherit oldDependency newDependency drv; }
-    ) baseSystem config.system.replaceRuntimeDependencies;
+    ) baseSystemAssertWarn config.system.replaceRuntimeDependencies;
 
 in