diff --git a/SignalServiceKit/src/Messages/OWSMessageSender.m b/SignalServiceKit/src/Messages/OWSMessageSender.m
index aaf54646e..59a581552 100644
--- a/SignalServiceKit/src/Messages/OWSMessageSender.m
+++ b/SignalServiceKit/src/Messages/OWSMessageSender.m
@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 #import "OWSMessageSender.h"
@@ -57,6 +57,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+NSString *NoSessionForTransientMessageException = @"NoSessionForTransientMessageException";
+
 const NSUInteger kOversizeTextMessageSizeThreshold = 2 * 1024;
 
 NSError *SSKEnsureError(NSError *_Nullable error, OWSErrorCode fallbackCode, NSString *fallbackErrorDescription)
@@ -858,7 +860,18 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
     @try {
         deviceMessages = [self throws_deviceMessagesForMessageSend:messageSend];
     } @catch (NSException *exception) {
-        if ([exception.name isEqualToString:UntrustedIdentityKeyException]) {
+        if ([exception.name isEqualToString:NoSessionForTransientMessageException]) {
+            // When users re-register, we don't want transient messages (like typing
+            // indicators) to cause users to hit the prekey fetch rate limit.  So
+            // we silently discard these message if there is no pre-existing session
+            // for the recipient.
+            NSError *error = OWSErrorWithCodeDescription(
+                OWSErrorCodeNoSessionForTransientMessage, @"No session for transient message.");
+            [error setIsRetryable:NO];
+            [error setIsFatal:YES];
+            *errorHandle = error;
+            return nil;
+        } else if ([exception.name isEqualToString:UntrustedIdentityKeyException]) {
             // This *can* happen under normal usage, but it should happen relatively rarely.
             // We expect it to happen whenever Bob reinstalls, and Alice messages Bob before
             // she can pull down his latest identity.
@@ -1548,6 +1561,11 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
     if (hasSession) {
         return;
     }
+    // Discard "typing indicator" messages if there is no existing session with the user.
+    BOOL canSafelyBeDiscarded = messageSend.message.isOnline;
+    if (canSafelyBeDiscarded) {
+        OWSRaiseException(NoSessionForTransientMessageException, @"No session for transient message.");
+    }
 
     __block dispatch_semaphore_t sema = dispatch_semaphore_create(0);
     __block PreKeyBundle *_Nullable bundle;
diff --git a/SignalServiceKit/src/Util/OWSError.h b/SignalServiceKit/src/Util/OWSError.h
index 238ab6b6d..5600d449c 100644
--- a/SignalServiceKit/src/Util/OWSError.h
+++ b/SignalServiceKit/src/Util/OWSError.h
@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 NS_ASSUME_NONNULL_BEGIN
@@ -51,6 +51,7 @@ typedef NS_ENUM(NSInteger, OWSErrorCode) {
     OWSErrorCodeProfileUpdateFailed = 777424,
     OWSErrorCodeAvatarWriteFailed = 777425,
     OWSErrorCodeAvatarUploadFailed = 777426,
+    OWSErrorCodeNoSessionForTransientMessage,
 };
 
 extern NSString *const OWSErrorRecipientIdentifierKey;