diff --git a/background/common.js b/background/common.js
index 87de1c10..d6e01fea 100644
--- a/background/common.js
+++ b/background/common.js
@@ -12,11 +12,20 @@
   uuidIndex
 */
 
-const uuidIndex = new Map();
 const bgReady = {};
 bgReady.styles = new Promise(r => (bgReady._resolveStyles = r));
 bgReady.all = new Promise(r => (bgReady._resolveAll = r));
 
+const uuidIndex = Object.assign(new Map(), {
+  custom: {},
+  customize(obj, setter) {
+    Object.defineProperty(uuidIndex.custom, obj.id, {
+      get: () => obj,
+      set: setter,
+    });
+  },
+});
+
 function addAPI(methods) {
   for (const [key, val] of Object.entries(methods)) {
     const old = API[key];
diff --git a/background/style-manager.js b/background/style-manager.js
index fc9de492..b480911c 100644
--- a/background/style-manager.js
+++ b/background/style-manager.js
@@ -73,10 +73,7 @@ const styleMan = (() => {
     _id: `${chrome.runtime.id}-${INJ_ORDER}`,
     _rev: 0,
   };
-  uuidIndex.custom = Object.defineProperty({}, INJ_ORDER, {
-    get: () => orderWrap,
-    set: setOrder,
-  });
+  uuidIndex.customize(orderWrap, setOrder);
   /** @type {Promise|boolean} will be `true` to avoid wasting a microtask tick on each `await` */
   let ready = init();