diff --git a/Signal/src/contact/SystemContactsFetcher.swift b/Signal/src/contact/SystemContactsFetcher.swift
index 29bf1bcba..79cc2ee8f 100644
--- a/Signal/src/contact/SystemContactsFetcher.swift
+++ b/Signal/src/contact/SystemContactsFetcher.swift
@@ -88,21 +88,30 @@ class SystemContactsFetcher: NSObject {
     }
 
     private func updateContacts() {
+        AssertIsOnMainThread()
+
         systemContactsHaveBeenRequestedAtLeastOnce = true
 
-        var systemContacts = [CNContact]()
-        do {
-            let contactFetchRequest = CNContactFetchRequest(keysToFetch: allowedContactKeys)
-            try contactStore.enumerateContacts(with: contactFetchRequest) { (contact, _) -> Void in
-                systemContacts.append(contact)
+        let contactStore = self.contactStore
+        let allowedContactKeys = self.allowedContactKeys
+
+        DispatchQueue.global().async {
+            var systemContacts = [CNContact]()
+            do {
+                let contactFetchRequest = CNContactFetchRequest(keysToFetch: allowedContactKeys)
+                try contactStore.enumerateContacts(with: contactFetchRequest) { (contact, _) -> Void in
+                    systemContacts.append(contact)
+                }
+            } catch let error as NSError {
+                Logger.error("\(self.TAG) Failed to fetch contacts with error:\(error)")
+                assertionFailure()
             }
-        } catch let error as NSError {
-            Logger.error("\(self.TAG) Failed to fetch contacts with error:\(error)")
-            assertionFailure()
-        }
 
-        let contacts = systemContacts.map { Contact(systemContact: $0) }
-        self.delegate?.systemContactsFetcher(self, updatedContacts: contacts)
+            let contacts = systemContacts.map { Contact(systemContact: $0) }
+            DispatchQueue.main.async {
+                self.delegate?.systemContactsFetcher(self, updatedContacts: contacts)
+            }
+        }
     }
 
     private func startObservingContactChanges() {
diff --git a/Signal/src/util/OWSContactsSyncing.m b/Signal/src/util/OWSContactsSyncing.m
index d59279a21..175bdf9e2 100644
--- a/Signal/src/util/OWSContactsSyncing.m
+++ b/Signal/src/util/OWSContactsSyncing.m
@@ -19,6 +19,8 @@ NSString *const kTSStorageManagerOWSContactsSyncingLastMessageKey =
 
 @interface OWSContactsSyncing ()
 
+@property (nonatomic, readonly) dispatch_queue_t serialQueue;
+
 @property (nonatomic, readonly) OWSContactsManager *contactsManager;
 @property (nonatomic, readonly) OWSMessageSender *messageSender;
 @property (nonatomic) BOOL isRequestInFlight;
@@ -70,50 +72,57 @@ NSString *const kTSStorageManagerOWSContactsSyncingLastMessageKey =
 {
     AssertIsOnMainThread();
 
-    if (self.isRequestInFlight) {
-        // De-bounce.  It's okay if we ignore some new changes;
-        // `sendSyncContactsMessageIfPossible` is called fairly
-        // often so we'll sync soon.
-        return;
+    if (!self.serialQueue) {
+        _serialQueue = dispatch_queue_create("org.whispersystems.contacts.syncing", DISPATCH_QUEUE_SERIAL);
     }
 
-    OWSSyncContactsMessage *syncContactsMessage =
-        [[OWSSyncContactsMessage alloc] initWithContactsManager:self.contactsManager];
-
-    NSData *messageData = [syncContactsMessage buildPlainTextAttachmentData];
-
-    NSData *lastMessageData =
-        [[TSStorageManager sharedManager] objectForKey:kTSStorageManagerOWSContactsSyncingLastMessageKey
-                                          inCollection:kTSStorageManagerOWSContactsSyncingCollection];
+    dispatch_async(self.serialQueue, ^{
 
-    if (lastMessageData && [lastMessageData isEqual:messageData]) {
-        // Ignore redundant contacts sync message.
-        return;
-    }
+        if (self.isRequestInFlight) {
+            // De-bounce.  It's okay if we ignore some new changes;
+            // `sendSyncContactsMessageIfPossible` is called fairly
+            // often so we'll sync soon.
+            return;
+        }
 
-    self.isRequestInFlight = YES;
+        OWSSyncContactsMessage *syncContactsMessage =
+            [[OWSSyncContactsMessage alloc] initWithContactsManager:self.contactsManager];
 
-    [self.messageSender sendTemporaryAttachmentData:[syncContactsMessage buildPlainTextAttachmentData]
-        contentType:OWSMimeTypeApplicationOctetStream
-        inMessage:syncContactsMessage
-        success:^{
-            DDLogInfo(@"%@ Successfully sent contacts sync message.", self.tag);
+        NSData *messageData = [syncContactsMessage buildPlainTextAttachmentData];
 
-            [[TSStorageManager sharedManager] setObject:messageData
-                                                 forKey:kTSStorageManagerOWSContactsSyncingLastMessageKey
-                                           inCollection:kTSStorageManagerOWSContactsSyncingCollection];
+        NSData *lastMessageData =
+            [[TSStorageManager sharedManager] objectForKey:kTSStorageManagerOWSContactsSyncingLastMessageKey
+                                              inCollection:kTSStorageManagerOWSContactsSyncingCollection];
 
-            dispatch_async(dispatch_get_main_queue(), ^{
-                self.isRequestInFlight = NO;
-            });
+        if (lastMessageData && [lastMessageData isEqual:messageData]) {
+            // Ignore redundant contacts sync message.
+            return;
         }
-        failure:^(NSError *error) {
-            DDLogError(@"%@ Failed to send contacts sync message with error: %@", self.tag, error);
 
-            dispatch_async(dispatch_get_main_queue(), ^{
-                self.isRequestInFlight = NO;
-            });
-        }];
+        self.isRequestInFlight = YES;
+
+        [self.messageSender sendTemporaryAttachmentData:[syncContactsMessage buildPlainTextAttachmentData]
+            contentType:OWSMimeTypeApplicationOctetStream
+            inMessage:syncContactsMessage
+            success:^{
+                DDLogInfo(@"%@ Successfully sent contacts sync message.", self.tag);
+
+                [[TSStorageManager sharedManager] setObject:messageData
+                                                     forKey:kTSStorageManagerOWSContactsSyncingLastMessageKey
+                                               inCollection:kTSStorageManagerOWSContactsSyncingCollection];
+
+                dispatch_async(self.serialQueue, ^{
+                    self.isRequestInFlight = NO;
+                });
+            }
+            failure:^(NSError *error) {
+                DDLogError(@"%@ Failed to send contacts sync message with error: %@", self.tag, error);
+
+                dispatch_async(self.serialQueue, ^{
+                    self.isRequestInFlight = NO;
+                });
+            }];
+    });
 }
 
 - (void)sendSyncContactsMessageIfPossible