diff --git a/src/write_coeff.cpp b/src/write_coeff.cpp
index 72b4835e55..83da6e4319 100644
--- a/src/write_coeff.cpp
+++ b/src/write_coeff.cpp
@@ -12,6 +12,7 @@
 ------------------------------------------------------------------------- */
 
 #include <cstring>
+#include <cstdlib>
 #include <unistd.h>
 #include "write_coeff.h"
 #include "pair.h"