diff --git a/modules/config/shells-environment.nix b/modules/config/shells-environment.nix
index fe72194a1ab..0bb7c4e26fe 100644
--- a/modules/config/shells-environment.nix
+++ b/modules/config/shells-environment.nix
@@ -25,7 +25,12 @@ in
       '';
       type = types.attrsOf (mkOptionType {
         name = "a string or a list of strings";
-        check = x: builtins.isString x || isList x;
+        merge = xs:
+          let xs' = evalProperties xs; in
+          if isList (head xs') then concatLists xs'
+          else if length xs' > 1 then abort "variable in ‘environment.variables’ has multiple values"
+          else if !builtins.isString (head xs') then abort "variable in ‘environment.variables’ does not have a string value"
+          else head xs';
       });
       apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
     };
diff --git a/tests/misc.nix b/tests/misc.nix
index 47775ce2a1e..e18a208fe43 100644
--- a/tests/misc.nix
+++ b/tests/misc.nix
@@ -8,6 +8,7 @@
     { config, pkgs, ... }:
     { swapDevices = pkgs.lib.mkOverride 0
         [ { device = "/root/swapfile"; size = 128; } ];
+      environment.variables.EDITOR = pkgs.lib.mkOverride 0 "emacs";
     };
 
   testScript =
@@ -52,6 +53,11 @@
       subtest "reboot-wtmp", sub {
           $machine->succeed("last | grep reboot >&2");
       };
+
+      # Test whether we can override environment variables.
+      subtest "override-env-var", sub {
+          $machine->succeed('[ "$EDITOR" = emacs ]');
+      };
     '';
 
 }