diff --git a/swaylock/seat.c b/swaylock/seat.c
index 6c46bb417..21db7c4fb 100644
--- a/swaylock/seat.c
+++ b/swaylock/seat.c
@@ -1,4 +1,5 @@
 #include <assert.h>
+#include <stdlib.h>
 #include <sys/mman.h>
 #include <unistd.h>
 #include <wlr/util/log.h>