diff --git a/src/create-handler-map.ts b/src/create-handler-map.ts
index e272349..e19845a 100644
--- a/src/create-handler-map.ts
+++ b/src/create-handler-map.ts
@@ -1,6 +1,7 @@
 import { ActionCreator } from './create-action'
 import { AnyAction } from './action'
 import { getType } from './get-type'
+import { Reducer } from './types'
 
 export type HandlerMap<State, Actions extends AnyAction> = {
   [key in Actions['type']]: Reducer<State, Actions>
diff --git a/src/types.d.ts b/src/types.d.ts
deleted file mode 100644
index 11b4763..0000000
--- a/src/types.d.ts
+++ /dev/null
@@ -1 +0,0 @@
-type Reducer<State, Actions> = (prevState: State, action: Actions) => State
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..52e8440
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,4 @@
+export type Reducer<State, Actions> = (
+  prevState: State,
+  action: Actions
+) => State
diff --git a/tsconfig.build.json b/tsconfig.build.json
index 787a229..eac0c2c 100644
--- a/tsconfig.build.json
+++ b/tsconfig.build.json
@@ -1,6 +1,6 @@
 {
   "extends": "./tsconfig.json",
-  "files": ["./src/index.ts", "./src/types.d.ts"],
+  "files": ["./src/index.ts"],
   "include": [],
   "exclude": [],
   "compilerOptions": {
diff --git a/tsconfig.spec.json b/tsconfig.spec.json
index 856bdb0..831918b 100644
--- a/tsconfig.spec.json
+++ b/tsconfig.spec.json
@@ -1,6 +1,6 @@
 {
   "extends": "./tsconfig.json",
-  "include": ["./src/**/__tests__/*", "./src/types.d.ts"],
+  "include": ["./src/**/__tests__/*"],
   "compilerOptions": {
     "target": "esnext",
     "esModuleInterop": true