diff --git a/rclcpp/include/rclcpp/callback_group.hpp b/rclcpp/include/rclcpp/callback_group.hpp
index 63ebca8..47ea417 100644
--- a/rclcpp/include/rclcpp/callback_group.hpp
+++ b/rclcpp/include/rclcpp/callback_group.hpp
@@ -66,7 +66,7 @@ public:
   get_service_ptrs() const;
 
   RCLCPP_PUBLIC
-  const std::vector<rclcpp::client::ClientBase::SharedPtr> &
+  const std::vector<rclcpp::client::ClientBase::WeakPtr> &
   get_client_ptrs() const;
 
   RCLCPP_PUBLIC
@@ -100,7 +100,7 @@ private:
   std::vector<rclcpp::subscription::SubscriptionBase::WeakPtr> subscription_ptrs_;
   std::vector<rclcpp::timer::TimerBase::WeakPtr> timer_ptrs_;
   std::vector<rclcpp::service::ServiceBase::SharedPtr> service_ptrs_;
-  std::vector<rclcpp::client::ClientBase::SharedPtr> client_ptrs_;
+  std::vector<rclcpp::client::ClientBase::WeakPtr> client_ptrs_;
   std::atomic_bool can_be_taken_from_;
 };
 
diff --git a/rclcpp/include/rclcpp/client.hpp b/rclcpp/include/rclcpp/client.hpp
index 2eaed00..3d27cb8 100644
--- a/rclcpp/include/rclcpp/client.hpp
+++ b/rclcpp/include/rclcpp/client.hpp
@@ -60,7 +60,7 @@ public:
   virtual std::shared_ptr<void> create_response() = 0;
   virtual std::shared_ptr<void> create_request_header() = 0;
   virtual void handle_response(
-    std::shared_ptr<void> & request_header, std::shared_ptr<void> & response) = 0;
+    std::shared_ptr<void> request_header, std::shared_ptr<void> response) = 0;
 
 private:
   RCLCPP_DISABLE_COPY(ClientBase);
@@ -111,13 +111,17 @@ public:
     return std::shared_ptr<void>(new rmw_request_id_t);
   }
 
-  void handle_response(std::shared_ptr<void> & request_header, std::shared_ptr<void> & response)
+  void handle_response(std::shared_ptr<void> request_header, std::shared_ptr<void> response)
   {
+    std::lock_guard<std::mutex> lock(pending_requests_mutex_);
     auto typed_request_header = std::static_pointer_cast<rmw_request_id_t>(request_header);
     auto typed_response = std::static_pointer_cast<typename ServiceT::Response>(response);
     int64_t sequence_number = typed_request_header->sequence_number;
-    // TODO(esteve) this must check if the sequence_number is valid otherwise the
-    // call_promise will be null
+    // TODO(esteve) this should throw instead since it is not expected to happen in the first place
+    if (this->pending_requests_.count(sequence_number) == 0) {
+      fprintf(stderr, "Received invalid sequence number. Ignoring...\n");
+      return;
+    }
     auto tuple = this->pending_requests_[sequence_number];
     auto call_promise = std::get<0>(tuple);
     auto callback = std::get<1>(tuple);
@@ -143,6 +147,7 @@ public:
   >
   SharedFuture async_send_request(SharedRequest request, CallbackT && cb)
   {
+    std::lock_guard<std::mutex> lock(pending_requests_mutex_);
     int64_t sequence_number;
     if (RMW_RET_OK != rmw_send_request(get_client_handle(), request.get(), &sequence_number)) {
       // *INDENT-OFF* (prevent uncrustify from making unecessary indents here)
@@ -187,6 +192,7 @@ private:
   RCLCPP_DISABLE_COPY(Client);
 
   std::map<int64_t, std::tuple<SharedPromise, CallbackType, SharedFuture>> pending_requests_;
+  std::mutex pending_requests_mutex_;
 };
 
 }  // namespace client
diff --git a/rclcpp/include/rclcpp/strategies/allocator_memory_strategy.hpp b/rclcpp/include/rclcpp/strategies/allocator_memory_strategy.hpp
index 6d14aca..d4d4975 100644
--- a/rclcpp/include/rclcpp/strategies/allocator_memory_strategy.hpp
+++ b/rclcpp/include/rclcpp/strategies/allocator_memory_strategy.hpp
@@ -151,7 +151,8 @@ public:
             services_.push_back(service);
           }
         }
-        for (auto & client : group->get_client_ptrs()) {
+        for (auto & weak_client : group->get_client_ptrs()) {
+          auto client = weak_client.lock();
           if (client) {
             clients_.push_back(client);
           }
diff --git a/rclcpp/src/rclcpp/callback_group.cpp b/rclcpp/src/rclcpp/callback_group.cpp
index 2555973..6d728a6 100644
--- a/rclcpp/src/rclcpp/callback_group.cpp
+++ b/rclcpp/src/rclcpp/callback_group.cpp
@@ -41,7 +41,7 @@ CallbackGroup::get_service_ptrs() const
   return service_ptrs_;
 }
 
-const std::vector<rclcpp::client::ClientBase::SharedPtr> &
+const std::vector<rclcpp::client::ClientBase::WeakPtr> &
 CallbackGroup::get_client_ptrs() const
 {
   return client_ptrs_;
diff --git a/rclcpp/src/rclcpp/memory_strategy.cpp b/rclcpp/src/rclcpp/memory_strategy.cpp
index 28c4bfd..a196fa3 100644
--- a/rclcpp/src/rclcpp/memory_strategy.cpp
+++ b/rclcpp/src/rclcpp/memory_strategy.cpp
@@ -84,8 +84,9 @@ MemoryStrategy::get_client_by_handle(void * client_handle, const WeakNodeVector
       if (!group) {
         continue;
       }
-      for (auto & client : group->get_client_ptrs()) {
-        if (client->get_client_handle()->data == client_handle) {
+      for (auto & weak_client : group->get_client_ptrs()) {
+        auto client = weak_client.lock();
+        if (client && client->get_client_handle()->data == client_handle) {
           return client;
         }
       }
@@ -182,8 +183,9 @@ MemoryStrategy::get_group_by_client(
       if (!group) {
         continue;
       }
-      for (auto & cli : group->get_client_ptrs()) {
-        if (cli == client) {
+      for (auto & weak_client : group->get_client_ptrs()) {
+        auto cli = weak_client.lock();
+        if (cli && cli == client) {
           return group;
         }
       }