diff --git a/src/index.ts b/src/index.ts
index ef0a2a5..dc0fbea 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,4 +2,4 @@ export * from './spy'
 export * from './spyOn'
 export * from './restoreAll'
 export { createInternalSpy, spies, getInternalState } from './internal'
-export type { Spy, SpyImpl } from './internal'
+export type { Spy, SpyImpl, SpyInternal, SpyInternalImpl } from './internal'
diff --git a/src/internal.ts b/src/internal.ts
index 8bed480..8355d81 100644
--- a/src/internal.ts
+++ b/src/internal.ts
@@ -1,6 +1,11 @@
 import { assert, define, defineValue, isPromise, isType } from './utils'
 import { S } from './constants'
 
+interface GetState {
+  <A extends any[], R>(spy: SpyInternalImpl<A, R>): SpyInternalImplState<A, R>
+  <A extends any[], R>(spy: SpyInternal<A, R>): SpyInternalState<A, R>
+}
+
 export let spies = new Set<SpyImpl>()
 
 let reset = (state: SpyInternalState) => {
@@ -13,8 +18,8 @@ let defineState = (spy: SpyInternal) => {
   define(spy, S, { value: { reset: () => reset(spy[S]) } })
   return spy[S]
 }
-export let getInternalState = <A extends any[], R>(spy: SpyInternal<A, R>) => {
-  return spy[S] || defineState(spy)
+export let getInternalState: GetState = (spy) => {
+  return (spy[S] || defineState(spy)) as any
 }
 
 type ReturnError = ['error', any]
diff --git a/src/spyOn.ts b/src/spyOn.ts
index 432a758..130ee67 100644
--- a/src/spyOn.ts
+++ b/src/spyOn.ts
@@ -23,7 +23,7 @@ type Constructors<T> = {
 let getDescriptor = (obj: any, method: string | symbol | number) =>
   Object.getOwnPropertyDescriptor(obj, method)
 
-export function internalSpyOn<T extends object, K extends string & keyof T>(
+export function internalSpyOn<T, K extends string & keyof T>(
   obj: T,
   methodName: K | { getter: K } | { setter: K },
   mock?: Procedure
diff --git a/src/utils.ts b/src/utils.ts
index 329587f..c1b6928 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,4 +1,4 @@
-export function assert(condition: any, message: string) {
+export function assert(condition: any, message: string): asserts condition {
   if (!condition) {
     throw new Error(message)
   }