diff --git a/AWSIoT/Internal/AWSIoTMQTTClient.m b/AWSIoT/Internal/AWSIoTMQTTClient.m
index 8cce64a6446..2f7b43dfa64 100644
--- a/AWSIoT/Internal/AWSIoTMQTTClient.m
+++ b/AWSIoT/Internal/AWSIoTMQTTClient.m
@@ -710,10 +710,15 @@ - (void)invalidateReconnectTimer {
 - (void)cleanUpWebsocketOutputStream {
     @synchronized(self) {
         if (self.websocketOutputStream) {
-            self.websocketOutputStream.delegate = nil;
-            [self.websocketOutputStream close];
-            [self.websocketOutputStream removeFromRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
-            self.websocketOutputStream = nil;
+            // This `websocketOutputStream` object here is possible to be accessed by other threads
+            @synchronized(self.websocketOutputStream) {
+                if (self.websocketOutputStream) { // We'd better double check its existence after obtained the lock
+                    self.websocketOutputStream.delegate = nil;
+                    [self.websocketOutputStream close];
+                    [self.websocketOutputStream removeFromRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
+                    self.websocketOutputStream = nil;
+                }
+            }
         }
     }
 }
@@ -1262,7 +1267,10 @@ - (void)webSocket:(AWSSRWebSocket *)webSocket didFailWithError:(NSError *)error
     // Also, the webSocket can be set to nil
     [self cleanUpWebsocketOutputStream];
 
-    [self.encoderOutputStream close];
+    // This `encoderOutputStream` object here is possible to be accessed by other threads
+    @synchronized (self.encoderOutputStream) {
+        [self.encoderOutputStream close];
+    }
     [self.webSocket close];
     self.webSocket = nil;
     
@@ -1300,7 +1308,10 @@ - (void)webSocket:(AWSSRWebSocket *)webSocket didCloseWithCode:(NSInteger)code r
     // The WebSocket has closed. The input/output streams can be closed here.
     [self cleanUpWebsocketOutputStream];
 
-    [self.encoderOutputStream close];
+    // This `encoderOutputStream` object here is possible to be accessed by other threads
+    @synchronized (self.encoderOutputStream) {
+        [self.encoderOutputStream close];
+    }
     [self.webSocket close];
     self.webSocket = nil;
     
diff --git a/AWSIoT/Internal/AWSIoTStreamThread.m b/AWSIoT/Internal/AWSIoTStreamThread.m
index 8ad47bcc445..9597761e1cf 100644
--- a/AWSIoT/Internal/AWSIoTStreamThread.m
+++ b/AWSIoT/Internal/AWSIoTStreamThread.m
@@ -150,25 +150,33 @@ - (void)cleanUp {
             }
 
             // Make sure we handle the streams in a thread-safe way
-            if (self.outputStream) {
-                // Remove from runLoop first before closing
-                if (self.runLoopForStreamsThread) {
-                    [self.outputStream removeFromRunLoop:self.runLoopForStreamsThread
-                                                 forMode:NSDefaultRunLoopMode];
+            @synchronized(self.outputStream) {
+                if (self.outputStream) {
+                    // Remove from runLoop first before closing
+                    if (self.runLoopForStreamsThread) {
+                        [self.outputStream removeFromRunLoop:self.runLoopForStreamsThread
+                                                    forMode:NSDefaultRunLoopMode];
+                    }
+                    self.outputStream.delegate = nil;
+                    [self.outputStream close];
+                    self.outputStream = nil;
                 }
-                self.outputStream.delegate = nil;
-                [self.outputStream close];
-                self.outputStream = nil;
             }
 
-            if (self.decoderInputStream) {
-                [self.decoderInputStream close];
-                self.decoderInputStream = nil;
+            // Make sure we handle the streams in a thread-safe way
+            @synchronized(self.decoderInputStream) {
+                if (self.decoderInputStream) {
+                    [self.decoderInputStream close];
+                    self.decoderInputStream = nil;
+                }
             }
 
-            if (self.encoderOutputStream) {
-                [self.encoderOutputStream close];
-                self.encoderOutputStream = nil;
+            // Make sure we handle the streams in a thread-safe way
+            @synchronized(self.encoderOutputStream) {
+                if (self.encoderOutputStream) {
+                    [self.encoderOutputStream close];
+                    self.encoderOutputStream = nil;
+                }
             }
         } else {
             AWSDDLogVerbose(@"Skipping disconnect for thread: [%@]", (NSThread *)self);
diff --git a/AWSIoT/Internal/MQTTSDK/AWSMQTTEncoder.m b/AWSIoT/Internal/MQTTSDK/AWSMQTTEncoder.m
index 9a6fc4986e7..9c396c89189 100644
--- a/AWSIoT/Internal/MQTTSDK/AWSMQTTEncoder.m
+++ b/AWSIoT/Internal/MQTTSDK/AWSMQTTEncoder.m
@@ -44,10 +44,15 @@ - (void)open {
 }
 
 - (void)close {
-    AWSDDLogDebug(@"closing encoder stream.");
-    [self.stream close];
-    [self.stream setDelegate:nil];
-    self.stream = nil;
+    // Make sure we handle the streams in a thread-safe way
+    @synchronized (self.stream) {
+        if (self.stream) { // We'd better double check that the stream is not nil
+            AWSDDLogDebug(@"closing encoder stream.");
+            [self.stream close];
+            [self.stream setDelegate:nil];
+            self.stream = nil;
+        }
+    }
 }
 
 //This is executed in the runLoop.