diff --git a/nixos/modules/system/boot/stage-1.nix b/nixos/modules/system/boot/stage-1.nix
index b2b662803728..7f7184b1e453 100644
--- a/nixos/modules/system/boot/stage-1.nix
+++ b/nixos/modules/system/boot/stage-1.nix
@@ -330,6 +330,11 @@ in
 
   config = {
 
+    assertions = singleton
+      { assertion = any (fs: fs.mountPoint == "/") (attrValues config.fileSystems);
+        message = "The ‘fileSystems’ option does not specify your root file system.";
+      };
+
     system.build.bootStage1 = bootStage1;
     system.build.initialRamdisk = initialRamdisk;
     system.build.extraUtils = extraUtils;
diff --git a/nixos/modules/tasks/filesystems.nix b/nixos/modules/tasks/filesystems.nix
index 3f484045ed4c..b0bcd2eb373e 100644
--- a/nixos/modules/tasks/filesystems.nix
+++ b/nixos/modules/tasks/filesystems.nix
@@ -81,6 +81,7 @@ in
   options = {
 
     fileSystems = mkOption {
+      default = {};
       example = {
         "/".device = "/dev/hda1";
         "/data" = {