diff --git a/clients/c/cosmopolite-int.h b/clients/c/cosmopolite-int.h
index c9973f5..6858d4b 100644
--- a/clients/c/cosmopolite-int.h
+++ b/clients/c/cosmopolite-int.h
@@ -7,6 +7,7 @@ struct cosmo_command {
   struct cosmo_command *prev;
   struct cosmo_command *next;
   json_t *command;
+  promise *promise;
 };
 
 struct cosmo {
diff --git a/clients/c/cosmopolite.c b/clients/c/cosmopolite.c
index 89e39e2..efa0239 100644
--- a/clients/c/cosmopolite.c
+++ b/clients/c/cosmopolite.c
@@ -87,18 +87,19 @@ static json_t *cosmo_find_subscription(cosmo *instance, json_t *subject) {
   return NULL;
 }
 
-static void cosmo_send_command_locked(cosmo *instance, json_t *command) {
+static void cosmo_send_command_locked(cosmo *instance, json_t *command, promise *promise_obj) {
   struct cosmo_command *command_obj = malloc(sizeof(*command_obj));
   command_obj->command = command;
+  command_obj->promise = promise_obj;
   cosmo_append_command(&instance->command_queue_head, &instance->command_queue_tail, command_obj);
   instance->next_delay_ms = 0;
 }
 
 // Takes ownership of command.
-static void cosmo_send_command(cosmo *instance, json_t *command) {
+static void cosmo_send_command(cosmo *instance, json_t *command, promise *promise_obj) {
   assert(command);
   assert(!pthread_mutex_lock(&instance->lock));
-  cosmo_send_command_locked(instance, command);
+  cosmo_send_command_locked(instance, command, promise_obj);
   assert(!pthread_cond_signal(&instance->cond));
   assert(!pthread_mutex_unlock(&instance->lock));
 }
@@ -400,7 +401,7 @@ static struct cosmo_command *cosmo_send_rpc(cosmo *instance, struct cosmo_comman
           }
         }
 
-        cosmo_send_command_locked(instance, cosmo_command("subscribe", arguments));
+        cosmo_send_command_locked(instance, cosmo_command("subscribe", arguments), NULL);
       }
     }
     assert(!pthread_mutex_unlock(&instance->lock));
@@ -557,7 +558,7 @@ void cosmo_subscribe(cosmo *instance, json_t *subjects, const json_int_t message
       json_object_set_new(arguments, "last_id", json_integer(last_id));
       json_object_set_new(subscription, "last_id", json_integer(last_id));
     }
-    cosmo_send_command_locked(instance, cosmo_command("subscribe", arguments));
+    cosmo_send_command_locked(instance, cosmo_command("subscribe", arguments), promise_obj);
   }
   assert(!pthread_mutex_unlock(&instance->lock));
 
@@ -579,7 +580,7 @@ void cosmo_unsubscribe(cosmo *instance, json_t *subject, promise *promise_obj) {
   assert(!pthread_mutex_unlock(&instance->lock));
 
   json_t *arguments = json_pack("{sO}", "subject", subject);
-  cosmo_send_command(instance, cosmo_command("unsubscribe", arguments));
+  cosmo_send_command(instance, cosmo_command("unsubscribe", arguments), promise_obj);
 }
 
 void cosmo_send_message(cosmo *instance, json_t *subject, json_t *message, promise *promise_obj) {
@@ -590,7 +591,7 @@ void cosmo_send_message(cosmo *instance, json_t *subject, json_t *message, promi
       "subject", subject,
       "message", encoded,
       "sender_message_id", sender_message_id);
-  cosmo_send_command(instance, cosmo_command("sendMessage", arguments));
+  cosmo_send_command(instance, cosmo_command("sendMessage", arguments), promise_obj);
   free(encoded);
 }