diff --git a/src/lib/libpthread.js b/src/lib/libpthread.js
index 6aeb5c409b185..d7007c7f1987e 100644
--- a/src/lib/libpthread.js
+++ b/src/lib/libpthread.js
@@ -286,11 +286,7 @@ var LibraryPThread = {
         // If this message is intended to a recipient that is not the main
         // thread, forward it to the target thread. This is currently only
         // used by `CMD_CHECK_MAILBOX`.
-        if (d.targetThread) {
-#if ASSERTIONS
-          // pthreads should not be relaying messages to themselves.
-          assert(d.targetThread != _pthread_self());
-#endif
+        if (d.targetThread && d.targetThread != _pthread_self()) {
           var targetWorker = PThread.pthreads[d.targetThread];
 #if ASSERTIONS
           if (!targetWorker) err(`worker sent message (${cmd}) to pthread (${d.targetThread}) that no longer exists`);
diff --git a/test/codesize/test_codesize_minimal_pthreads.json b/test/codesize/test_codesize_minimal_pthreads.json
index 7394016e2d40f..ab8c2d4354a6e 100644
--- a/test/codesize/test_codesize_minimal_pthreads.json
+++ b/test/codesize/test_codesize_minimal_pthreads.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 6945,
-  "a.out.js.gz": 3424,
+  "a.out.js": 6955,
+  "a.out.js.gz": 3431,
   "a.out.nodebug.wasm": 19108,
   "a.out.nodebug.wasm.gz": 8824,
-  "total": 26053,
-  "total_gz": 12248,
+  "total": 26063,
+  "total_gz": 12255,
   "sent": [
     "a (memory)",
     "b (exit)",
diff --git a/test/codesize/test_codesize_minimal_pthreads_memgrowth.json b/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
index 6b72597e5813f..56a3d321836d4 100644
--- a/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
+++ b/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 7379,
-  "a.out.js.gz": 3639,
+  "a.out.js": 7389,
+  "a.out.js.gz": 3647,
   "a.out.nodebug.wasm": 19109,
   "a.out.nodebug.wasm.gz": 8826,
-  "total": 26488,
-  "total_gz": 12465,
+  "total": 26498,
+  "total_gz": 12473,
   "sent": [
     "a (memory)",
     "b (exit)",
diff --git a/test/test_browser.py b/test/test_browser.py
index aafa29e9dcb5b..f2b70ee312b66 100644
--- a/test/test_browser.py
+++ b/test/test_browser.py
@@ -3954,6 +3954,12 @@ def test_gauge_available_memory(self, args):
   def test_pthread_run_on_main_thread(self):
     self.btest_exit('pthread/test_pthread_run_on_main_thread.c', cflags=['-O3', '-pthread', '-sPTHREAD_POOL_SIZE'])
 
+  # Test that proxying operations work when Atomics.waitAsync is disabled,
+  # forcing the waitAsyncPolyfilled/postMessage fallback path.
+  def test_pthread_no_waitasync(self):
+    create_file('pre.js', 'delete Atomics.waitAsync;\n')
+    self.btest_exit('pthread/test_pthread_proxy_to_pthread.c', cflags=['-O3', '-pthread', '-sPROXY_TO_PTHREAD', '-sASSERTIONS', '--pre-js=pre.js'])
+
   # Test how a lot of back-to-back called proxying operations behave.
   def test_pthread_run_on_main_thread_flood(self):
     self.btest_exit('pthread/test_pthread_run_on_main_thread_flood.c', cflags=['-O3', '-pthread', '-sPTHREAD_POOL_SIZE'])