diff --git a/libc/include/pthread.h.def b/libc/include/pthread.h.def
--- a/libc/include/pthread.h.def
+++ b/libc/include/pthread.h.def
@@ -13,6 +13,7 @@
 
 #define PTHREAD_STACK_MIN (1 << 14) // 16KB
 
+#define PTHREAD_MUTEX_INITIALIZER {0}
 #define PTHREAD_ONCE_INIT {0}
 
 enum {