diff --git a/src/when.js b/src/when.js
index 30e9ef7..5dba5b3 100644
--- a/src/when.js
+++ b/src/when.js
@@ -84,9 +84,10 @@ class WhenMock {
 
       this.nextCallMockId++
 
-      this.fn.mockImplementation((...args) => {
-        for (let i = 0; i < this.callMocks.length; i++) {
-          const { matchers, mockImplementation, expectCall, once, called } = this.callMocks[i]
+      const instance = this
+      this.fn.mockImplementation(function (...args) {
+        for (let i = 0; i < instance.callMocks.length; i++) {
+          const { matchers, mockImplementation, expectCall, once, called } = instance.callMocks[i]
 
           // Do not let a once mock match more than once
           if (once && called) continue
@@ -110,16 +111,16 @@ class WhenMock {
           }
 
           if (isMatch && typeof mockImplementation === 'function') {
-            this.callMocks[i].called = true
-            return mockImplementation(...args)
+            instance.callMocks[i].called = true
+            return mockImplementation.call(this, ...args)
           }
         }
 
-        if (this._defaultImplementation) {
-          return this._defaultImplementation(...args)
+        if (instance._defaultImplementation) {
+          return instance._defaultImplementation.call(this, ...args)
         }
         if (typeof fn.__whenMock__._origMock === 'function') {
-          return fn.__whenMock__._origMock(...args)
+          return fn.__whenMock__._origMock.call(this, ...args)
         }
         return undefined
       })
diff --git a/src/when.test.js b/src/when.test.js
index 1761fd0..7455553 100644
--- a/src/when.test.js
+++ b/src/when.test.js
@@ -1025,6 +1025,54 @@ describe('When', () => {
       expect(fn(2)).toEqual('b')
     })
 
+    it('keeps call context when not matched', () => {
+      class TheClass {
+        call () {
+          return 'ok'
+        }
+
+        request (...args) {
+          return this.call(...args)
+        }
+      }
+
+      const theInstance = new TheClass()
+
+      const theSpiedMethod = jest.spyOn(theInstance, 'request')
+
+      when(theSpiedMethod)
+        .calledWith(1)
+        .mockReturnValue('mock')
+
+      const unhandledCall = theInstance.request()
+      expect(unhandledCall).toBe('ok')
+    })
+
+    it('keeps call context when matched', () => {
+      class TheClass {
+        call () {
+          return 'ok'
+        }
+
+        request (...args) {
+          return this.call(...args)
+        }
+      }
+
+      const theInstance = new TheClass()
+
+      const theSpiedMethod = jest.spyOn(theInstance, 'request')
+
+      when(theSpiedMethod)
+        .calledWith(1)
+        .mockImplementation(function () {
+          return this.call() + '!'
+        })
+
+      const unhandledCall = theInstance.request(1)
+      expect(unhandledCall).toBe('ok!')
+    })
+
     it('does not add to the number of assertion calls', () => {
       expect.assertions(0)
     })