diff --git a/src/lib/thread/compat_threads.c b/src/lib/thread/compat_threads.c
index 1d685b2c3e..972960e242 100644
--- a/src/lib/thread/compat_threads.c
+++ b/src/lib/thread/compat_threads.c
@@ -18,6 +18,8 @@
 #include "lib/log/torlog.h"
 #include "lib/log/util_bug.h"
 
+#include <string.h>
+
 /** Allocate and return a new condition variable. */
 tor_cond_t *
 tor_cond_new(void)