diff --git a/src/env-inl.h b/src/env-inl.h
index 917ddd1b6bc..f115656353c 100644
--- a/src/env-inl.h
+++ b/src/env-inl.h
@@ -371,6 +371,15 @@ inline void Environment::CloseHandle(T* handle, OnCloseCallback callback) {
   });
 }
 
+void Environment::IncreaseWaitingRequestCounter() {
+  request_waiting_++;
+}
+
+void Environment::DecreaseWaitingRequestCounter() {
+  request_waiting_--;
+  CHECK_GE(request_waiting_, 0);
+}
+
 inline uv_loop_t* Environment::event_loop() const {
   return isolate_data()->event_loop();
 }
diff --git a/src/env.cc b/src/env.cc
index 6526c680ac1..e5b9c0fd6aa 100644
--- a/src/env.cc
+++ b/src/env.cc
@@ -107,7 +107,6 @@ Environment::Environment(IsolateData* isolate_data,
 #if HAVE_INSPECTOR
       inspector_agent_(new inspector::Agent(this)),
 #endif
-      handle_cleanup_waiting_(0),
       http_parser_buffer_(nullptr),
       fs_stats_field_array_(isolate_, kFsStatsFieldsLength * 2),
       context_(context->GetIsolate(), context) {
@@ -241,8 +240,11 @@ void Environment::CleanupHandles() {
     hc.cb_(this, hc.handle_, hc.arg_);
   handle_cleanup_queue_.clear();
 
-  while (handle_cleanup_waiting_ != 0 || !handle_wrap_queue_.IsEmpty())
+  while (handle_cleanup_waiting_ != 0 ||
+         request_waiting_ != 0 ||
+         !handle_wrap_queue_.IsEmpty()) {
     uv_run(event_loop(), UV_RUN_ONCE);
+  }
 }
 
 void Environment::StartProfilerIdleNotifier() {
diff --git a/src/env.h b/src/env.h
index 79351666c11..de3014249ea 100644
--- a/src/env.h
+++ b/src/env.h
@@ -601,6 +601,9 @@ class Environment {
   inline uv_check_t* immediate_check_handle();
   inline uv_idle_t* immediate_idle_handle();
 
+  inline void IncreaseWaitingRequestCounter();
+  inline void DecreaseWaitingRequestCounter();
+
   inline AsyncHooks* async_hooks();
   inline ImmediateInfo* immediate_info();
   inline TickInfo* tick_info();
@@ -833,7 +836,8 @@ class Environment {
   HandleWrapQueue handle_wrap_queue_;
   ReqWrapQueue req_wrap_queue_;
   std::list<HandleCleanup> handle_cleanup_queue_;
-  int handle_cleanup_waiting_;
+  int handle_cleanup_waiting_ = 0;
+  int request_waiting_ = 0;
 
   double* heap_statistics_buffer_ = nullptr;
   double* heap_space_statistics_buffer_ = nullptr;
diff --git a/src/node_api.cc b/src/node_api.cc
index d5437d70d93..91a47a12d92 100644
--- a/src/node_api.cc
+++ b/src/node_api.cc
@@ -3388,6 +3388,9 @@ class Work : public node::AsyncResource {
       // Establish a handle scope here so that every callback doesn't have to.
       // Also it is needed for the exception-handling below.
       v8::HandleScope scope(env->isolate);
+      node::Environment* env_ = node::Environment::GetCurrent(env->isolate);
+      env_->DecreaseWaitingRequestCounter();
+
       CallbackScope callback_scope(work);
 
       NAPI_CALL_INTO_MODULE(env,
@@ -3488,6 +3491,8 @@ napi_status napi_queue_async_work(napi_env env, napi_async_work work) {
 
   uvimpl::Work* w = reinterpret_cast<uvimpl::Work*>(work);
 
+  node::Environment* env_ = node::Environment::GetCurrent(env->isolate);
+  env_->IncreaseWaitingRequestCounter();
   CALL_UV(env, uv_queue_work(event_loop,
                              w->Request(),
                              uvimpl::Work::ExecuteCallback,
diff --git a/src/node_crypto.cc b/src/node_crypto.cc
index f611f81f16a..10e4f593914 100644
--- a/src/node_crypto.cc
+++ b/src/node_crypto.cc
@@ -4639,9 +4639,12 @@ void PBKDF2Request::After() {
 
 
 void PBKDF2Request::After(uv_work_t* work_req, int status) {
-  CHECK_EQ(status, 0);
   std::unique_ptr<PBKDF2Request> req(
       ContainerOf(&PBKDF2Request::work_req_, work_req));
+  req->env()->DecreaseWaitingRequestCounter();
+  if (status == UV_ECANCELED)
+    return;
+  CHECK_EQ(status, 0);
   req->After();
 }
 
@@ -4692,6 +4695,7 @@ void PBKDF2(const FunctionCallbackInfo<Value>& args) {
   if (args[5]->IsFunction()) {
     obj->Set(env->context(), env->ondone_string(), args[5]).FromJust();
 
+    env->IncreaseWaitingRequestCounter();
     uv_queue_work(env->event_loop(),
                   req.release()->work_req(),
                   PBKDF2Request::Work,
@@ -4831,10 +4835,13 @@ void RandomBytesCheck(RandomBytesRequest* req, Local<Value> (*argv)[2]) {
 
 
 void RandomBytesAfter(uv_work_t* work_req, int status) {
-  CHECK_EQ(status, 0);
   std::unique_ptr<RandomBytesRequest> req(
       ContainerOf(&RandomBytesRequest::work_req_, work_req));
   Environment* env = req->env();
+  env->DecreaseWaitingRequestCounter();
+  if (status == UV_ECANCELED)
+    return;
+  CHECK_EQ(status, 0);
   HandleScope handle_scope(env->isolate());
   Context::Scope context_scope(env->context());
   Local<Value> argv[2];
@@ -4874,6 +4881,7 @@ void RandomBytes(const FunctionCallbackInfo<Value>& args) {
   if (args[1]->IsFunction()) {
     obj->Set(env->context(), env->ondone_string(), args[1]).FromJust();
 
+    env->IncreaseWaitingRequestCounter();
     uv_queue_work(env->event_loop(),
                   req.release()->work_req(),
                   RandomBytesWork,
@@ -4913,6 +4921,7 @@ void RandomBytesBuffer(const FunctionCallbackInfo<Value>& args) {
   if (args[3]->IsFunction()) {
     obj->Set(env->context(), env->ondone_string(), args[3]).FromJust();
 
+    env->IncreaseWaitingRequestCounter();
     uv_queue_work(env->event_loop(),
                   req.release()->work_req(),
                   RandomBytesWork,
diff --git a/src/node_zlib.cc b/src/node_zlib.cc
index ec447638e2a..3249905dfbf 100644
--- a/src/node_zlib.cc
+++ b/src/node_zlib.cc
@@ -214,6 +214,7 @@ class ZCtx : public AsyncWrap {
     }
 
     // async version
+    env->IncreaseWaitingRequestCounter();
     uv_queue_work(env->event_loop(), work_req, ZCtx::Process, ZCtx::After);
   }
 
@@ -361,10 +362,17 @@ class ZCtx : public AsyncWrap {
 
   // v8 land!
   static void After(uv_work_t* work_req, int status) {
-    CHECK_EQ(status, 0);
-
     ZCtx* ctx = ContainerOf(&ZCtx::work_req_, work_req);
     Environment* env = ctx->env();
+    ctx->write_in_progress_ = false;
+
+    env->DecreaseWaitingRequestCounter();
+    if (status == UV_ECANCELED) {
+      ctx->Close();
+      return;
+    }
+
+    CHECK_EQ(status, 0);
 
     HandleScope handle_scope(env->isolate());
     Context::Scope context_scope(env->context());
@@ -374,7 +382,6 @@ class ZCtx : public AsyncWrap {
 
     ctx->write_result_[0] = ctx->strm_.avail_out;
     ctx->write_result_[1] = ctx->strm_.avail_in;
-    ctx->write_in_progress_ = false;
 
     // call the write() cb
     Local<Function> cb = PersistentToLocal(env->isolate(),
diff --git a/src/req_wrap-inl.h b/src/req_wrap-inl.h
index 54abf74430f..e3b26c1f5c6 100644
--- a/src/req_wrap-inl.h
+++ b/src/req_wrap-inl.h
@@ -20,6 +20,8 @@ ReqWrap<T>::ReqWrap(Environment* env,
   // FIXME(bnoordhuis) The fact that a reinterpret_cast is needed is
   // arguably a good indicator that there should be more than one queue.
   env->req_wrap_queue()->PushBack(reinterpret_cast<ReqWrap<uv_req_t>*>(this));
+
+  Reset();
 }
 
 template <typename T>
@@ -33,6 +35,12 @@ void ReqWrap<T>::Dispatched() {
   req_.data = this;
 }
 
+template <typename T>
+void ReqWrap<T>::Reset() {
+  original_callback_ = nullptr;
+  req_.data = nullptr;
+}
+
 template <typename T>
 ReqWrap<T>* ReqWrap<T>::from_req(T* req) {
   return ContainerOf(&ReqWrap<T>::req_, req);
@@ -40,7 +48,8 @@ ReqWrap<T>* ReqWrap<T>::from_req(T* req) {
 
 template <typename T>
 void ReqWrap<T>::Cancel() {
-  uv_cancel(reinterpret_cast<uv_req_t*>(&req_));
+  if (req_.data == this)  // Only cancel if already dispatched.
+    uv_cancel(reinterpret_cast<uv_req_t*>(&req_));
 }
 
 // Below is dark template magic designed to invoke libuv functions that
@@ -95,7 +104,7 @@ struct CallLibuvFunction<ReqT, void(*)(ReqT*, Args...)> {
 template <typename ReqT, typename T>
 struct MakeLibuvRequestCallback {
   static T For(ReqWrap<ReqT>* req_wrap, T v) {
-    static_assert(!std::is_function<T>::value,
+    static_assert(!is_callable<T>::value,
                   "MakeLibuvRequestCallback missed a callback");
     return v;
   }
@@ -109,6 +118,7 @@ struct MakeLibuvRequestCallback<ReqT, void(*)(ReqT*, Args...)> {
 
   static void Wrapper(ReqT* req, Args... args) {
     ReqWrap<ReqT>* req_wrap = ContainerOf(&ReqWrap<ReqT>::req_, req);
+    req_wrap->env()->DecreaseWaitingRequestCounter();
     F original_callback = reinterpret_cast<F>(req_wrap->original_callback_);
     original_callback(req, args...);
   }
@@ -128,23 +138,26 @@ int ReqWrap<T>::Dispatch(LibuvFunction fn, Args... args) {
 
   // This expands as:
   //
-  // return fn(env()->event_loop(), req(), arg1, arg2, Wrapper, arg3, ...)
-  //           ^                                       ^        ^
-  //           |                                       |        |
-  //           \-- Omitted if `fn` has no              |        |
-  //               first `uv_loop_t*` argument         |        |
-  //                                                   |        |
-  //     A function callback whose first argument      |        |
-  //     matches the libuv request type is replaced ---/        |
-  //     by the `Wrapper` method defined above                  |
-  //                                                            |
-  //            Other (non-function) arguments are passed  -----/
-  //            through verbatim
-  return CallLibuvFunction<T, LibuvFunction>::Call(
+  // int err = fn(env()->event_loop(), req(), arg1, arg2, Wrapper, arg3, ...)
+  //              ^                                       ^        ^
+  //              |                                       |        |
+  //              \-- Omitted if `fn` has no              |        |
+  //                  first `uv_loop_t*` argument         |        |
+  //                                                      |        |
+  //        A function callback whose first argument      |        |
+  //        matches the libuv request type is replaced ---/        |
+  //        by the `Wrapper` method defined above                  |
+  //                                                               |
+  //               Other (non-function) arguments are passed  -----/
+  //               through verbatim
+  int err = CallLibuvFunction<T, LibuvFunction>::Call(
       fn,
       env()->event_loop(),
       req(),
       MakeLibuvRequestCallback<T, Args>::For(this, args)...);
+  if (err >= 0)
+    env()->IncreaseWaitingRequestCounter();
+  return err;
 }
 
 }  // namespace node
diff --git a/src/req_wrap.h b/src/req_wrap.h
index d1818172182..8f8d0cf2885 100644
--- a/src/req_wrap.h
+++ b/src/req_wrap.h
@@ -20,6 +20,8 @@ class ReqWrap : public AsyncWrap {
   // Call this after the req has been dispatched, if that did not already
   // happen by using Dispatch().
   inline void Dispatched();
+  // Call this after a request has finished, if re-using this object is planned.
+  inline void Reset();
   T* req() { return &req_; }
   inline void Cancel();
 
diff --git a/src/util.h b/src/util.h
index 7a1c6c109fd..2c66104e9d2 100644
--- a/src/util.h
+++ b/src/util.h
@@ -447,8 +447,16 @@ struct MallocedBuffer {
   MallocedBuffer& operator=(const MallocedBuffer&) = delete;
 };
 
-}  // namespace node
+// Test whether some value can be called with ().
+template<typename T, typename = void>
+struct is_callable : std::is_function<T> { };
 
+template<typename T>
+struct is_callable<T, typename std::enable_if<
+    std::is_same<decltype(void(&T::operator())), void>::value
+    >::type> : std::true_type { };
+
+}  // namespace node
 
 #endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS