diff --git a/src/USER-MISC/fix_ipi.cpp b/src/USER-MISC/fix_ipi.cpp
index 6b7d06cbd9..07dbeb2500 100644
--- a/src/USER-MISC/fix_ipi.cpp
+++ b/src/USER-MISC/fix_ipi.cpp
@@ -18,6 +18,7 @@
 #include "mpi.h"
 #include "stdio.h"
 #include "string.h"
+#include "stdlib.h"
 #include "fix_ipi.h"
 #include "atom.h"
 #include "force.h"