diff --git a/src/libecoprimer/smothsort.c b/src/libecoprimer/smothsort.c
index 72ee444..e48f262 100644
--- a/src/libecoprimer/smothsort.c
+++ b/src/libecoprimer/smothsort.c
@@ -35,6 +35,7 @@
  * @author Pekka Pessi <Pekka.Pessi@nokia.com>
  */
 
+#include <stdlib.h> /* FB for NULL */
 
 #include <assert.h>
 #include <stdio.h>