diff --git a/system/lib/libc/musl/include/unistd.h b/system/lib/libc/musl/include/unistd.h
index 635fd4abf4bde..902f4281260b2 100644
--- a/system/lib/libc/musl/include/unistd.h
+++ b/system/lib/libc/musl/include/unistd.h
@@ -258,7 +258,11 @@ pid_t gettid(void);
 
 #define _POSIX_VDISABLE         0
 
+#if defined(__EMSCRIPTEN__) && !defined(_REENTRANT) /* XXX Emscripten doesn't always support pthreads */
+#define _POSIX_THREADS          -1
+#else
 #define _POSIX_THREADS          _POSIX_VERSION
+#endif
 #define _POSIX_THREAD_PROCESS_SHARED _POSIX_VERSION
 #define _POSIX_THREAD_SAFE_FUNCTIONS _POSIX_VERSION
 #define _POSIX_THREAD_ATTR_STACKADDR _POSIX_VERSION