diff --git a/clients/c/cosmopolite.c b/clients/c/cosmopolite.c
index 7978038..c9e2190 100644
--- a/clients/c/cosmopolite.c
+++ b/clients/c/cosmopolite.c
@@ -87,6 +87,17 @@ static json_t *cosmo_find_subscription(cosmo *instance, json_t *subject) {
   return NULL;
 }
 
+void cosmo_remove_subscription(cosmo *instance, json_t *subject) {
+  size_t i;
+  json_t *subscription;
+  json_array_foreach(instance->subscriptions, i, subscription) {
+    if (json_equal(json_object_get(subscription, "subject"), subject)) {
+      json_array_remove(instance->subscriptions, i);
+      break;
+    }
+  }
+}
+
 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;
@@ -304,6 +315,54 @@ static void cosmo_handle_event(cosmo *instance, json_t *event) {
   }
 }
 
+static void cosmo_complete_subscribe(cosmo *instance, struct cosmo_command *command, json_t *response, char *result) {
+  json_t *subject;
+  assert(!json_unpack(command->command, "{s{so}}", "arguments", "subject", &subject));
+
+  if (strcmp(result, "ok")) {
+    cosmo_remove_subscription(instance, subject);
+    promise_fail(command->promise, NULL);
+    return;
+  }
+
+  assert(!pthread_mutex_lock(&instance->lock));
+  json_t *subscription = cosmo_find_subscription(instance, subject);
+  if (subscription) {
+    // Might have unsubscribed later
+    json_object_set_new(subscription, "state", json_integer(SUBSCRIPTION_ACTIVE));
+  }
+  assert(!pthread_mutex_unlock(&instance->lock));
+
+  promise_succeed(command->promise, NULL);
+}
+
+static void cosmo_complete_unsubscribe(cosmo *instance, struct cosmo_command *command, json_t *response, char *result) {
+  promise_complete(command->promise, NULL, (strcmp(result, "ok") == 0));
+}
+
+static void cosmo_complete_send_message(cosmo *instance, struct cosmo_command *command, json_t *response, char *result) {
+  json_t *message;
+  int err = json_unpack(response, "{so}", "message", &message);
+  if (err || strcmp(result, "ok")) {
+    promise_fail(command->promise, NULL);
+  } else {
+    promise_succeed(command->promise, message);
+  }
+}
+
+static void cosmo_complete_rpc(cosmo *instance, struct cosmo_command *command, json_t *response) {
+  char *command_name, *result;
+  assert(!json_unpack(command->command, "{ss}", "command", &command_name));
+  assert(!json_unpack(response, "{ss}", "result", &result));
+  if (!strcmp(command_name, "subscribe")) {
+    cosmo_complete_subscribe(instance, command, response, result);
+  } else if (!strcmp(command_name, "unsubscribe")) {
+    cosmo_complete_unsubscribe(instance, command, response, result);
+  } else if (!strcmp(command_name, "sendMessage")) {
+    cosmo_complete_send_message(instance, command, response, result);
+  }
+}
+
 // Takes ownership of arguments.
 static json_t *cosmo_command(const char *name, const json_t *arguments) {
   return json_pack("{ssso}", "command", name, "arguments", arguments);
@@ -440,19 +499,7 @@ static struct cosmo_command *cosmo_send_rpc(cosmo *instance, struct cosmo_comman
       continue;
     }
 
-    char *command_name;
-    assert(!json_unpack(command_iter->command, "{ss}", "command", &command_name));
-    if (!strcmp(command_name, "subscribe")) {
-      json_t *subject = NULL;
-      assert(!json_unpack(command_iter->command, "{s{so}}", "arguments", "subject", &subject));
-      assert(!pthread_mutex_lock(&instance->lock));
-      json_t *subscription = cosmo_find_subscription(instance, subject);
-      if (subscription) {
-        // Might have unsubscribed later
-        json_object_set_new(subscription, "state", json_integer(SUBSCRIPTION_ACTIVE));
-      }
-      assert(!pthread_mutex_unlock(&instance->lock));
-    }
+    cosmo_complete_rpc(instance, command_iter, command_response);
 
     json_decref(command_iter->command);
     free(command_iter);
@@ -574,14 +621,7 @@ void cosmo_subscribe(cosmo *instance, json_t *subjects, const json_int_t message
 
 void cosmo_unsubscribe(cosmo *instance, json_t *subject, promise *promise_obj) {
   assert(!pthread_mutex_lock(&instance->lock));
-  size_t i;
-  json_t *subscription;
-  json_array_foreach(instance->subscriptions, i, subscription) {
-    if (json_equal(json_object_get(subscription, "subject"), subject)) {
-      json_array_remove(instance->subscriptions, i);
-      break;
-    }
-  }
+  cosmo_remove_subscription(instance, subject);
   assert(!pthread_mutex_unlock(&instance->lock));
 
   json_t *arguments = json_pack("{sO}", "subject", subject);
diff --git a/clients/c/promise.c b/clients/c/promise.c
index a5b1848..a980c29 100644
--- a/clients/c/promise.c
+++ b/clients/c/promise.c
@@ -38,6 +38,7 @@ static void promise_destroy(promise *promise_obj) {
 }
 
 bool promise_wait(promise *promise_obj, void **result) {
+  assert(promise_obj);
   assert(!pthread_mutex_lock(&promise_obj->lock));
   assert(promise_obj->will_wait);
   while (!promise_obj->fulfilled) {
@@ -55,6 +56,10 @@ bool promise_wait(promise *promise_obj, void **result) {
 }
 
 void promise_complete(promise *promise_obj, void *result, bool success) {
+  if (!promise_obj) {
+    return;
+  }
+
   assert(!pthread_mutex_lock(&promise_obj->lock));
 
   if (success && promise_obj->on_success) {