diff --git a/users/lib.nix b/users/lib.nix
index 245ac4a..6e3eec8 100644
--- a/users/lib.nix
+++ b/users/lib.nix
@@ -1,19 +1,17 @@
 { lib }:
-
 let
   chrs = lib.listToAttrs (lib.imap (i: v: { name = v; value = i + 96; }) lib.lowerChars);
   ord = c: builtins.getAttr c chrs;
-
 in
 {
   # Make a unique UID from a 4-char identifier
-  mkUid = id: let
-    # TODO: Assert length
-    chars = lib.stringToCharacters id;
-    n = builtins.map (c: lib.mod (ord c) 10) chars;
-    s = builtins.concatStringsSep "" (builtins.map (i: builtins.toString i) n);
-
-  in
-    assert lib.length chars == 4;
-    1000 + lib.toInt s;
+  mkUid = id:
+    let
+      chars = lib.stringToCharacters (builtins.substring 0 4 id);
+      n = builtins.map (c: lib.mod (ord c) 10) chars;
+      s = builtins.concatStringsSep "" (builtins.map (i: builtins.toString i) n);
+    in
+      assert builtins.stringLength id >= 4;
+      assert builtins.length chars == 4;
+      1000 + lib.toInt s;
 }