diff --git a/src/once.ts b/src/once.ts
index a69b646..b9b2975 100644
--- a/src/once.ts
+++ b/src/once.ts
@@ -1,3 +1,5 @@
+const instances = new WeakMap<any, WeakMap<any, any>>();
+
 /**
  * A class method decorator that limits a method to be called only once. All
  * subsequent calls will return the result of the first call.
@@ -6,10 +8,9 @@ export function once<This, Args extends any[], Return>(
   fn: (this: This, ...args: Args) => Return,
   context?: ClassMethodDecoratorContext<This, (this: This, ...args: Args) => Return>,
 ) {
-  const instances = new WeakMap<any, WeakMap<any, any>>();
   return function (this: This, ...args: Args): Return {
-    if (!instances.has(this)) instances.set(this, new WeakMap<any, any>());
-    const map = instances.get(this)!;
+    let map = instances.get(this);
+    if (!map) instances.set(this, map = new WeakMap<any, any>());
     if (!map.has(fn)) map.set(fn, fn.apply(this, args));
     return map.get(fn);
   };