diff --git a/rktwebview.cpp b/rktwebview.cpp
index 2073aa0..7dcf49b 100644
--- a/rktwebview.cpp
+++ b/rktwebview.cpp
@@ -87,6 +87,7 @@ typedef struct {
     std::string log_file;
 
     void (*evt_cb)(int entries);
+    std::mutex evt_cb_mutex;
     MemQueue *evt_queue;
     std::thread *guard_thread;
     bool guard_go_on;
@@ -226,6 +227,8 @@ bool runRktWebview(Handle_t *handler)
 
 #define EVT_GUARD_STOP -93273
 #define EVT_ALIVE_ERROR -94328
+#define EVT_NULL -95372
+
 #define WAIT_ON_EVENT_MS (2 * 1000)
 #define ALIVE_MESSAGE_INTERVAL_S 5                  // Should be smaller than 10 seconds
 #define MAX_WAIT_RESULT     (2 * 1000)               // Maximum wait in milliseconds for a result per time
@@ -241,15 +244,20 @@ void rkt_evt_guard(void)
             if (handler->verbose) {
                 INFO2("Got event wv = %d, %s\n", wv, data.c_str());
             }
-            if (wv != EVT_GUARD_STOP) {
+            if (wv != EVT_GUARD_STOP && wv != EVT_NULL) {
                 handler->evt_queue->enqueue(wv, data);
+                handler->evt_cb_mutex.lock();
                 if (handler->evt_cb != nullptr) {       // evt_cb could be cleared to null
                     INFO2("Calling evt_cb with %d, %s\n", wv, data.c_str());
+
                     handler->evt_cb(1);
                     INFO0("done\n");
                 } else {
                     INFO0("evt_cb = null\n");
                 }
+                handler->evt_cb_mutex.unlock();
+            } else if (wv == EVT_NULL) {
+                INFO0("Got NULL message (evt_guard)\n");
             } else {
                 INFO0("Got stop message (evt_guard)\n");
                 handler->guard_go_on = false;
@@ -261,6 +269,7 @@ void rkt_evt_guard(void)
         }
     }
     INFO0("Leaving rkt_evt_guard\n");
+
 }
 
 
@@ -303,9 +312,15 @@ void rkt_webview_cleanup()
         INFO0("rkt_webview_cleanup - cleaning up\n");
 
         handler->verbose = true;
+        handler->event_queue->enqueue(EVT_NULL);
+        //sleep_ms(1000);     // Why is this second so important?
+        // I just don't get why the evt_guard thread is blocking and getting core dumped
+        // on the thread join, if this second is not given
+        // (to racket?).
 
         if (handler->valid) {
             if (handler->guard_thread != nullptr) {
+                handler->evt_cb_mutex.lock();
                 if (handler->evt_cb != nullptr) {
                     INFO0("Clearing evt_guard callback to null\n");
                     handler->evt_cb = nullptr;
@@ -314,6 +329,7 @@ void rkt_webview_cleanup()
                 }
                 INFO0("Stopping evt_guard (enqueuing evt_guard_stop)\n");
                 handler->event_queue->enqueue(EVT_GUARD_STOP);
+                handler->evt_cb_mutex.unlock();
                 INFO0("yielding current thread and hopefully activating evt_guard thread\n");
                 YIELD();
                 INFO0("joining guard thread\n");
@@ -449,14 +465,25 @@ static inline bool validHandle()
 #define FAIL_CALL_JS if (!validHandle()) { return nullptr; }
 #define FAIL_INFO FAIL_CALL_JS
 
-void rkt_webview_register_evt_callback(void (*f)(int))
+int rkt_webview_register_evt_callback(void (*f)(int))
 {
     RKT_WEBVIEW_INIT;
 
-    handler->evt_cb = f;
-    if (handler->guard_thread == nullptr) {
-        handler->guard_go_on = true;
-        handler->guard_thread = new std::thread(rkt_evt_guard);
+    if (handler->evt_cb_mutex.try_lock()) {
+        INFO1("registering callback %p\n", f);
+
+        handler->evt_cb = f;
+        if (handler->guard_thread == nullptr) {
+            handler->guard_go_on = true;
+            handler->guard_thread = new std::thread(rkt_evt_guard);
+            INFO0("guard thread started\n");
+        }
+
+        handler->evt_cb_mutex.unlock();
+        return 1;
+    } else {
+        WARN1("Cannot aquire lock for registering callback %p\n", f);
+        return 0;
     }
 }
 
diff --git a/rktwebview.h b/rktwebview.h
index 96de054..20be273 100644
--- a/rktwebview.h
+++ b/rktwebview.h
@@ -14,7 +14,7 @@ RKTWEBVIEW_EXPORT void rkt_webview_env(const char *env_cmds[]);
 RKTWEBVIEW_EXPORT void rkt_webview_init(const char *from);
 RKTWEBVIEW_EXPORT void rkt_webview_cleanup();
 RKTWEBVIEW_EXPORT void rkt_webview_exit_done(int done);
-RKTWEBVIEW_EXPORT void rkt_webview_register_evt_callback(void (*f)(int));
+RKTWEBVIEW_EXPORT int rkt_webview_register_evt_callback(void (*f)(int));
 
 RKTWEBVIEW_EXPORT void rkt_webview_set_loglevel(rkt_webview_loglevel_t l);
 RKTWEBVIEW_EXPORT rkt_data_t *rkt_webview_info();