Allow chaining of any http.Handler, not just http.HandlerFunc.

README.md

diff --git a/README.md b/README.md
index 9875c70..785a108 100644
--- a/README.md
+++ b/README.md
@@ -297,7 +297,7 @@
 
 **NOTE: It might be required to set [Router.HandleMethodNotAllowed](http://godoc.org/github.com/julienschmidt/httprouter#Router.HandleMethodNotAllowed) to `false` to avoid problems.**
 
-You can use another [http.HandlerFunc](http://golang.org/pkg/net/http/#HandlerFunc), for example another router, to handle requests which could not be matched by this router by using the [Router.NotFound](http://godoc.org/github.com/julienschmidt/httprouter#Router.NotFound) handler. This allows chaining.
+You can use another [http.Handler](http://golang.org/pkg/net/http/#Handler), for example another router, to handle requests which could not be matched by this router by using the [Router.NotFound](http://godoc.org/github.com/julienschmidt/httprouter#Router.NotFound) handler. This allows chaining.
 
 ### Static files
 The `NotFound` handler can for example be used to serve static files from the root path `/` (like an index.html file along with other assets):

router.go

diff --git a/router.go b/router.go
index 155b871..83e86fd 100644
--- a/router.go
+++ b/router.go
@@ -138,14 +138,14 @@
 	// handler.
 	HandleMethodNotAllowed bool
 
-	// Configurable http.HandlerFunc which is called when no matching route is
+	// Configurable http.Handler which is called when no matching route is
 	// found. If it is not set, http.NotFound is used.
-	NotFound http.HandlerFunc
+	NotFound http.Handler
 
-	// Configurable http.HandlerFunc which is called when a request
+	// Configurable http.Handler which is called when a request
 	// cannot be routed and HandleMethodNotAllowed is true.
 	// If it is not set, http.Error with http.StatusMethodNotAllowed is used.
-	MethodNotAllowed http.HandlerFunc
+	MethodNotAllowed http.Handler
 
 	// Function to handle panics recovered from http handlers.
 	// It should be used to generate a error page and return the http error code
@@ -342,7 +342,7 @@
 			handle, _, _ := r.trees[method].getValue(req.URL.Path)
 			if handle != nil {
 				if r.MethodNotAllowed != nil {
-					r.MethodNotAllowed(w, req)
+					r.MethodNotAllowed.ServeHTTP(w, req)
 				} else {
 					http.Error(w,
 						http.StatusText(http.StatusMethodNotAllowed),
@@ -356,7 +356,7 @@
 
 	// Handle 404
 	if r.NotFound != nil {
-		r.NotFound(w, req)
+		r.NotFound.ServeHTTP(w, req)
 	} else {
 		http.NotFound(w, req)
 	}

router_test.go

diff --git a/router_test.go b/router_test.go
index 9dc6296..e3141bd 100644
--- a/router_test.go
+++ b/router_test.go
@@ -174,6 +174,48 @@
 	}
 }
 
+func TestRouterChaining(t *testing.T) {
+	router1 := New()
+	router2 := New()
+	router1.NotFound = router2
+
+	fooHit := false
+	router1.POST("/foo", func(w http.ResponseWriter, req *http.Request, _ Params) {
+		fooHit = true
+		w.WriteHeader(http.StatusOK)
+	})
+
+	barHit := false
+	router2.POST("/bar", func(w http.ResponseWriter, req *http.Request, _ Params) {
+		barHit = true
+		w.WriteHeader(http.StatusOK)
+	})
+
+	r, _ := http.NewRequest("POST", "/foo", nil)
+	w := httptest.NewRecorder()
+	router1.ServeHTTP(w, r)
+	if !(w.Code == http.StatusOK && fooHit) {
+		t.Errorf("Regular routing failed with router chaining.")
+		t.FailNow()
+	}
+
+	r, _ = http.NewRequest("POST", "/bar", nil)
+	w = httptest.NewRecorder()
+	router1.ServeHTTP(w, r)
+	if !(w.Code == http.StatusOK && barHit) {
+		t.Errorf("Chained routing failed with router chaining.")
+		t.FailNow()
+	}
+
+	r, _ = http.NewRequest("POST", "/qax", nil)
+	w = httptest.NewRecorder()
+	router1.ServeHTTP(w, r)
+	if !(w.Code == http.StatusNotFound) {
+		t.Errorf("NotFound behavior failed with router chaining.")
+		t.FailNow()
+	}
+}
+
 func TestRouterNotAllowed(t *testing.T) {
 	handlerFunc := func(_ http.ResponseWriter, _ *http.Request, _ Params) {}
 
@@ -190,10 +232,10 @@
 
 	w = httptest.NewRecorder()
 	responseText := "custom method"
-	router.MethodNotAllowed = func(w http.ResponseWriter, req *http.Request) {
+	router.MethodNotAllowed = http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		w.WriteHeader(http.StatusTeapot)
 		w.Write([]byte(responseText))
-	}
+	})
 	router.ServeHTTP(w, r)
 	if got := w.Body.String(); !(got == responseText) {
 		t.Errorf("unexpected response got %q want %q", got, responseText)
@@ -237,10 +279,10 @@
 
 	// Test custom not found handler
 	var notFound bool
-	router.NotFound = func(rw http.ResponseWriter, r *http.Request) {
+	router.NotFound = http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
 		rw.WriteHeader(404)
 		notFound = true
-	}
+	})
 	r, _ := http.NewRequest("GET", "/nope", nil)
 	w := httptest.NewRecorder()
 	router.ServeHTTP(w, r)