diff --git a/README.md b/README.md
index 8536c28..54edb05 100644
--- a/README.md
+++ b/README.md
@@ -186,7 +186,7 @@ export const calculateQuestion = async (answer: number): Promise<string> => {
 
 ### `when(spy: TFunc, options?: WhenOptions): StubWrapper<TFunc>`
 
-Configures a `vi.fn()` mock function to act as a vitest-when stub. Adds an implementation to the function that initially no-ops, and returns an API to configure behaviors for given arguments using [`.calledWith(...)`][called-with]
+Configures a `vi.fn()` or `vi.spyOn()` mock function to act as a vitest-when stub. Adds an implementation to the function that initially no-ops, and returns an API to configure behaviors for given arguments using [`.calledWith(...)`][called-with]
 
 ```ts
 import { vi } from 'vitest'
@@ -264,6 +264,21 @@ when(overloaded).calledWith().thenReturn(null)
 when<() => null>(overloaded).calledWith().thenReturn(null)
 ```
 
+#### Fallback
+
+By default, if arguments do not match, a vitest-when stub will no-op and return `undefined`. You can customize this fallback by configuring your own unconditional behavior on the mock using Vitest's built-in [mock API][].
+
+```ts
+const spy = vi.fn().mockReturnValue('you messed up!')
+
+when(spy).calledWith('hello').thenReturn('world')
+
+spy('hello') // "world"
+spy('jello') // "you messed up!"
+```
+
+[mock API]: https://vitest.dev/api/mock.html
+
 ### `.thenReturn(value: TReturn)`
 
 When the stubbing is satisfied, return `value`
diff --git a/src/debug.ts b/src/debug.ts
index f5f7d84..851063b 100644
--- a/src/debug.ts
+++ b/src/debug.ts
@@ -4,7 +4,7 @@ import {
 } from 'pretty-format'
 
 import { validateSpy, getBehaviorStack } from './stubs'
-import type { AnyFunction } from './types'
+import type { AnyFunction, MockInstance } from './types'
 import { type Behavior, BehaviorType } from './behaviors'
 
 export interface DebugResult {
@@ -21,11 +21,11 @@ export interface Stubbing {
 }
 
 export const getDebug = <TFunc extends AnyFunction>(
-  spy: TFunc,
+  spy: TFunc | MockInstance<TFunc>,
 ): DebugResult => {
-  const target = validateSpy(spy)
+  const target = validateSpy<TFunc>(spy)
   const name = target.getMockName()
-  const behaviors = getBehaviorStack<TFunc>(target)
+  const behaviors = getBehaviorStack(target)
   const unmatchedCalls = behaviors?.getUnmatchedCalls() ?? target.mock.calls
   const stubbings =
     behaviors?.getAll().map((entry) => ({
diff --git a/src/stubs.ts b/src/stubs.ts
index ea0186b..b1fe5a1 100644
--- a/src/stubs.ts
+++ b/src/stubs.ts
@@ -16,7 +16,7 @@ interface WhenStubImplementation<TFunc extends AnyFunction> {
 export const configureStub = <TFunc extends AnyFunction>(
   maybeSpy: unknown,
 ): BehaviorStack<TFunc> => {
-  const spy = validateSpy(maybeSpy)
+  const spy = validateSpy<TFunc>(maybeSpy)
   const existingBehaviors = getBehaviorStack(spy)
 
   if (existingBehaviors) {
@@ -24,11 +24,12 @@ export const configureStub = <TFunc extends AnyFunction>(
   }
 
   const behaviors = createBehaviorStack<TFunc>()
+  const fallbackImplementation = spy.getMockImplementation()
 
   const implementation = (...args: Parameters<TFunc>) => {
     const behavior = behaviors.use(args)?.behavior ?? {
-      type: BehaviorType.RETURN,
-      value: undefined,
+      type: BehaviorType.DO,
+      callback: fallbackImplementation,
     }
 
     switch (behavior.type) {
@@ -50,19 +51,21 @@ export const configureStub = <TFunc extends AnyFunction>(
       }
 
       case BehaviorType.DO: {
-        return behavior.callback(...args)
+        return behavior.callback?.(...args)
       }
     }
   }
 
   spy.mockImplementation(
-    Object.assign(implementation, { [BEHAVIORS_KEY]: behaviors }),
+    Object.assign(implementation as TFunc, { [BEHAVIORS_KEY]: behaviors }),
   )
 
   return behaviors
 }
 
-export const validateSpy = (maybeSpy: unknown): MockInstance => {
+export const validateSpy = <TFunc extends AnyFunction>(
+  maybeSpy: unknown,
+): MockInstance<TFunc> => {
   if (
     typeof maybeSpy === 'function' &&
     'mockImplementation' in maybeSpy &&
@@ -72,14 +75,14 @@ export const validateSpy = (maybeSpy: unknown): MockInstance => {
     'getMockName' in maybeSpy &&
     typeof maybeSpy.getMockName === 'function'
   ) {
-    return maybeSpy as unknown as MockInstance
+    return maybeSpy as unknown as MockInstance<TFunc>
   }
 
   throw new NotAMockFunctionError(maybeSpy)
 }
 
 export const getBehaviorStack = <TFunc extends AnyFunction>(
-  spy: MockInstance,
+  spy: MockInstance<TFunc>,
 ): BehaviorStack<TFunc> | undefined => {
   const existingImplementation = spy.getMockImplementation() as
     | WhenStubImplementation<TFunc>
diff --git a/src/vitest-when.ts b/src/vitest-when.ts
index 0577b08..4a1eb95 100644
--- a/src/vitest-when.ts
+++ b/src/vitest-when.ts
@@ -47,7 +47,7 @@ export interface DebugOptions {
 }
 
 export const debug = <TFunc extends AnyFunction>(
-  spy: TFunc,
+  spy: TFunc | MockInstance<TFunc>,
   options: DebugOptions = {},
 ): DebugResult => {
   const log = options.log ?? true
diff --git a/test/vitest-when.test.ts b/test/vitest-when.test.ts
index 4f3ae86..87283d2 100644
--- a/test/vitest-when.test.ts
+++ b/test/vitest-when.test.ts
@@ -55,6 +55,15 @@ describe('vitest-when', () => {
     expect(spy(1, 2, 3)).toEqual(undefined)
   })
 
+  it('should fall back to original mock implementation', () => {
+    const spy = vi.fn().mockReturnValue(100)
+
+    subject.when(spy).calledWith(1, 2, 3).thenReturn(4)
+
+    expect(spy(1, 2, 3)).toEqual(4)
+    expect(spy()).toEqual(100)
+  })
+
   it('should return a number of times', () => {
     const spy = vi.fn()