diff --git a/src/memoize.ts b/src/memoize.ts
index 2964c7ad39..a191f876c7 100644
--- a/src/memoize.ts
+++ b/src/memoize.ts
@@ -1,21 +1,22 @@
 import pLimit = require('p-limit')
 
-interface CachedPromises<T> {
-  [name: string]: Promise<T>
-}
-
 export type MemoizedFunc<T> = (key: string, fn: () => Promise<T>) => Promise<T>
 
 /**
  * Save promises for later
  */
 export default function memoize <T> (concurrency?: number): MemoizedFunc<T> {
-  const locks: CachedPromises<T> = {}
-  const limit = concurrency && pLimit(concurrency)
+  const locks = new Map<string, Promise<T>>()
+  const limit = concurrency && pLimit(concurrency) as (fn: () => Promise<T>) => Promise<T>
 
   return (key: string, fn: () => Promise<T>): Promise<T> => {
-    if (locks[key]) return locks[key]
-    locks[key] = limit && limit(fn) || fn()
-    return locks[key]
+    let v = locks.get(key)
+    if (v) return v
+    v = limit && limit(fn) || fn()
+    locks.set(key, v)
+    return v.catch((err: Error) => {
+      locks.delete(key)
+      throw err
+    })
   }
 }