diff --git a/src/api/become-observed.ts b/src/api/become-observed.ts
index 370787469..4d5f2d551 100644
--- a/src/api/become-observed.ts
+++ b/src/api/become-observed.ts
@@ -47,8 +47,8 @@ export function onBecomeUnobserved(thing, arg2, arg3?): Lambda {
 
 function interceptHook(hook: "onBecomeObserved" | "onBecomeUnobserved", thing, arg2, arg3) {
     const atom: IObservable =
-        typeof arg2 === "string" ? getAtom(thing, arg2) : (getAtom(thing) as any)
-    const cb = typeof arg2 === "string" ? arg3 : arg2
+        typeof arg3 === "function" ? getAtom(thing, arg2) : (getAtom(thing) as any)
+    const cb = typeof arg3 === "function" ? arg3 : arg2
     const orig = atom[hook]
 
     if (typeof orig !== "function")
diff --git a/test/base/become-observed.js b/test/base/become-observed.js
new file mode 100644
index 000000000..a282fe22d
--- /dev/null
+++ b/test/base/become-observed.js
@@ -0,0 +1,13 @@
+import { autorun, onBecomeObserved, observable } from "../../src/mobx"
+
+describe("become-observed", () => {
+    it("work on map with number as key", () => {
+        const oMap = observable.map()
+        const key = 1
+        oMap.set(key, observable.box("value"))
+        const cb = jest.fn()
+        onBecomeObserved(oMap, key, cb)
+        autorun(() => oMap.get(key))
+        expect(cb).toBeCalled()
+    })
+})