diff --git a/Signal/src/ViewControllers/DebugUI/DebugUIContacts.m b/Signal/src/ViewControllers/DebugUI/DebugUIContacts.m
index a77549a58..dbf72c28a 100644
--- a/Signal/src/ViewControllers/DebugUI/DebugUIContacts.m
+++ b/Signal/src/ViewControllers/DebugUI/DebugUIContacts.m
@@ -5,6 +5,7 @@
 #import "DebugUIContacts.h"
 #import "OWSTableViewController.h"
 #import "Signal-Swift.h"
+#import "SignalApp.h"
 #import <Contacts/Contacts.h>
 
 NS_ASSUME_NONNULL_BEGIN
@@ -50,6 +51,18 @@ NS_ASSUME_NONNULL_BEGIN
                                                            actionBlock:^{
                                                                [DebugUIContacts clearSignalAccountCache];
                                                            }],
+                                           [OWSTableItem itemWithTitle:@"Clear SignalRecipient Cache"
+                                                           actionBlock:^{
+                                                               [DebugUIContacts clearSignalRecipientCache];
+                                                           }],
+                                           [OWSTableItem itemWithTitle:@"New Invalid Contact Thread"
+                                                           actionBlock:^{
+                                                               [DebugUIContacts createInvalidContactThread];
+                                                           }],
+                                           [OWSTableItem itemWithTitle:@"New Invalid Group Thread"
+                                                           actionBlock:^{
+                                                               [DebugUIContacts createInvalidGroupThread];
+                                                           }],
                                        ]];
 }
 
@@ -1290,6 +1303,12 @@ NS_ASSUME_NONNULL_BEGIN
     [SignalAccount removeAllObjectsInCollection];
 }
 
++ (void)clearSignalRecipientCache
+{
+    DDLogWarn(@"%@ Deleting all signal recipients.", self.logTag);
+    [SignalRecipient removeAllObjectsInCollection];
+}
+
 + (void)deleteAllContacts
 {
     [self deleteContactsWithFilter:^(CNContact *contact) {
@@ -1304,6 +1323,41 @@ NS_ASSUME_NONNULL_BEGIN
     }];
 }
 
++ (NSString *)invalidRecipientId
+{
+    // We ensure that the phone number is invalid by appending too many digits.
+    NSMutableString *recipientId = [@"+1" mutableCopy];
+    for (int i = 0; i < 11; i++) {
+        [recipientId appendFormat:@"%d", (int)(arc4random() % 10)];
+    }
+    return [recipientId copy];
+}
+
++ (void)createInvalidContactThread
+{
+    NSString *recipientId = [self invalidRecipientId];
+    TSContactThread *thread = [TSContactThread getOrCreateThreadWithContactId:recipientId];
+    [SignalApp.sharedApp presentConversationForThread:thread];
+}
+
++ (void)createInvalidGroupThread
+{
+    NSString *invalidRecipientId = [self invalidRecipientId];
+    NSString *validRecipientId = @"+19174054216";
+
+    NSString *groupName = @"Partially invalid group";
+    NSMutableArray<NSString *> *recipientIds = [@[
+        invalidRecipientId,
+        validRecipientId,
+        [TSAccountManager localNumber],
+    ] mutableCopy];
+    NSData *groupId = [SecurityUtils generateRandomBytes:16];
+    TSGroupModel *model =
+        [[TSGroupModel alloc] initWithTitle:groupName memberIds:recipientIds image:nil groupId:groupId];
+    TSGroupThread *thread = [TSGroupThread getOrCreateThreadWithGroupModel:model];
+    [SignalApp.sharedApp presentConversationForThread:thread];
+}
+
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/SignalServiceKit/src/Messages/OWSMessageSender.m b/SignalServiceKit/src/Messages/OWSMessageSender.m
index ea2f506e4..cf183a70b 100644
--- a/SignalServiceKit/src/Messages/OWSMessageSender.m
+++ b/SignalServiceKit/src/Messages/OWSMessageSender.m
@@ -904,8 +904,6 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         } else if (!mayHaveLinkedDevices && hasDeviceMessages) {
             OWSFail(@"%@ sync message has device messages for unknown secondary devices.", self.logTag);
         }
-    } else {
-        OWSAssert(deviceMessages.count > 0);
     }
 
     if (deviceMessages.count == 0) {
@@ -914,6 +912,9 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         // * The first (after upgrading?) time we send a sync message to our linked devices.
         // * After unlinking all linked devices.
         // * After trying and failing to link a device.
+        // * The first time we send a message to a user, if they don't have their
+        //   default device (device id = 0).  For example, if they have unregistered
+        //   their primary but still have a linked device. Or later, when they re-register.
         //
         // When we're not sure if we have linked devices, we need to try
         // to send self-sync messages even if they have no device messages