diff --git a/internal/ossl/thread_setup_unix.c b/internal/ossl/thread_setup_unix.c
index 4447671..deac8ff 100644
--- a/internal/ossl/thread_setup_unix.c
+++ b/internal/ossl/thread_setup_unix.c
@@ -3,6 +3,7 @@
 #include "api.h"
 #include "thread_setup.h"
 #include <pthread.h>
+#include <stdlib.h>
 
 /* This array will store all of the mutexes available to OpenSSL. */ 
 static pthread_mutex_t *mutex_buf = NULL;