diff --git a/src/buildUpreducer/index.js b/src/buildUpreducer/index.js
deleted file mode 100644
index 52724f6..0000000
--- a/src/buildUpreducer/index.js
+++ /dev/null
@@ -1,15 +0,0 @@
-import fp from 'lodash/fp';
-
-export default function buildUpreducer(initial, mutations) {
-  return (action = {}) => (state) => {
-    if (state === null) state = initial;
-
-    const a =
-      mutations[(action).type] ||
-      mutations['*'];
-
-    if(!a) return state;
-
-    return a((action).payload, action)(state);
-  };
-}
diff --git a/src/buildUpreducer/index.ts b/src/buildUpreducer/index.ts
new file mode 100644
index 0000000..bedc8b2
--- /dev/null
+++ b/src/buildUpreducer/index.ts
@@ -0,0 +1,19 @@
+import fp from 'lodash/fp';
+
+import { Dictionary, Mutation, UpduxAction } from '../types';
+
+function buildUpreducer<S>(initial: S, mutations: Dictionary<Mutation<S>> ) {
+  return (action :UpduxAction) => (state: S) => {
+    if (state === null) state = initial;
+
+    const a =
+      mutations[action.type] ||
+      mutations['*'];
+
+    if(!a) return state;
+
+    return a(action.payload, action)(state);
+  };
+}
+
+export default buildUpreducer;
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..414d299
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,10 @@
+import { Action } from 'redux';
+
+export type UpduxAction = Action & Partial<{
+    payload: any,
+    meta: any,
+}>
+
+export type Dictionary<T> = { [key: string]: T };
+
+export type Mutation<S> = (payload: any, action: Action) => (state: S) => S ;