diff --git a/nixos/lib/testing.nix b/nixos/lib/testing.nix
index 7670687710363..e33ee8c6ba7d0 100644
--- a/nixos/lib/testing.nix
+++ b/nixos/lib/testing.nix
@@ -188,6 +188,7 @@ in rec {
             networking.hostName = "client";
             nix.readOnlyStore = false;
             virtualisation.writableStore = false;
+            system.stateVersion = "19.09"; # TODO: determine automatically
           }
         ];
 
diff --git a/nixos/modules/misc/version.nix b/nixos/modules/misc/version.nix
index 3ae60cb791601..54009f214ac83 100644
--- a/nixos/modules/misc/version.nix
+++ b/nixos/modules/misc/version.nix
@@ -49,17 +49,20 @@ in
 
     stateVersion = mkOption {
       type = types.str;
-      default = cfg.release;
+      # Deliberately no default, as automatically upgrading the stateVersion can
+      # lead systems to stop working.
+      example = cfg.release;
       description = ''
-        Every once in a while, a new NixOS release may change
-        configuration defaults in a way incompatible with stateful
-        data. For instance, if the default version of PostgreSQL
-        changes, the new version will probably be unable to read your
-        existing databases. To prevent such breakage, you can set the
-        value of this option to the NixOS release with which you want
-        to be compatible. The effect is that NixOS will option
-        defaults corresponding to the specified release (such as using
-        an older version of PostgreSQL).
+        This option exists to prevent breakage for NixOS modules that depend on
+        state which can't be controlled by them. The value of this option
+        represents the <em>initial</em> NixOS version installed, and should
+        therefore <em>not</em> be ever changed unless you make sure to run all
+        necessary migrations yourself. By fixing this value, NixOS modules can
+        know what version your state is in, such that they can make incompatible
+        changes for new installations, without breaking the module for all
+        existing installations. For example this is used in the postgresql
+        module, where a new module version uses a newer postgresql version which
+        can't read the databases of older versions.
       '';
     };