diff --git a/system/lib/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h b/system/lib/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
index 769fe36a90f78..27ee8dc70d513 100644
--- a/system/lib/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
+++ b/system/lib/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
@@ -427,7 +427,8 @@ struct __sanitizer_file_handle {
 };
 #endif
 
-#if SANITIZER_MAC
+// These fields are not actually pointers, and so wasm64 must use unsigned and not uptr for them
+#if SANITIZER_MAC || SANITIZER_EMSCRIPTEN
 struct __sanitizer_msghdr {
   void *msg_name;
   unsigned msg_namelen;
diff --git a/system/lib/compiler-rt/lib/ubsan/ubsan_flags.cpp b/system/lib/compiler-rt/lib/ubsan/ubsan_flags.cpp
index 48bb217d45695..d5463b1fc8607 100644
--- a/system/lib/compiler-rt/lib/ubsan/ubsan_flags.cpp
+++ b/system/lib/compiler-rt/lib/ubsan/ubsan_flags.cpp
@@ -75,6 +75,10 @@ void InitializeFlags() {
   parser.ParseString(__ubsan_default_options());
   // Override from environment variable.
 #if SANITIZER_EMSCRIPTEN
+#ifdef __wasm64__
+    // FIXME: support UBSAN in wasm64.
+    abort();
+#else
   char *options = (char*) EM_ASM_INT({
     return withBuiltinMalloc(function () {
       return allocateUTF8(Module['UBSAN_OPTIONS'] || 0);
@@ -82,6 +86,7 @@ void InitializeFlags() {
   });
   parser.ParseString(options);
   emscripten_builtin_free(options);
+#endif
 #else
   parser.ParseStringFromEnv("UBSAN_OPTIONS");
 #endif // SANITIZER_EMSCRIPTEN