diff --git a/src/lib/libpthread.js b/src/lib/libpthread.js
index c9ba01e4820d6..ac112b6a6dc80 100644
--- a/src/lib/libpthread.js
+++ b/src/lib/libpthread.js
@@ -278,14 +278,18 @@ var LibraryPThread = {
 #endif
 
         // If this message is intended to a recipient that is not the main
-        // thread, forward it to the target thread.
-        if (d.targetThread && d.targetThread != _pthread_self()) {
+        // 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
           var targetWorker = PThread.pthreads[d.targetThread];
-          if (targetWorker) {
-            targetWorker.postMessage(d, d.transferList);
-          } else {
-            err(`worker sent message (${cmd}) to pthread (${d.targetThread}) that no longer exists`);
-          }
+#if ASSERTIONS
+          if (!targetWorker) err(`worker sent message (${cmd}) to pthread (${d.targetThread}) that no longer exists`);
+#endif
+          targetWorker?.postMessage(d);
           return;
         }
 
diff --git a/test/codesize/test_codesize_minimal_pthreads.json b/test/codesize/test_codesize_minimal_pthreads.json
index 851b17340ee79..7843e557c59d8 100644
--- a/test/codesize/test_codesize_minimal_pthreads.json
+++ b/test/codesize/test_codesize_minimal_pthreads.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 7110,
-  "a.out.js.gz": 3524,
+  "a.out.js": 7012,
+  "a.out.js.gz": 3472,
   "a.out.nodebug.wasm": 19037,
   "a.out.nodebug.wasm.gz": 8787,
-  "total": 26147,
-  "total_gz": 12311,
+  "total": 26049,
+  "total_gz": 12259,
   "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 9dcfca60775f6..079165820445c 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": 7518,
-  "a.out.js.gz": 3728,
+  "a.out.js": 7420,
+  "a.out.js.gz": 3672,
   "a.out.nodebug.wasm": 19038,
   "a.out.nodebug.wasm.gz": 8788,
-  "total": 26556,
-  "total_gz": 12516,
+  "total": 26458,
+  "total_gz": 12460,
   "sent": [
     "a (memory)",
     "b (exit)",