diff --git a/src/internfile/mh_mbox.cpp b/src/internfile/mh_mbox.cpp
index 3051382f..cb241b7b 100644
--- a/src/internfile/mh_mbox.cpp
+++ b/src/internfile/mh_mbox.cpp
@@ -525,6 +525,7 @@ using namespace std;
 
 #include "rclconfig.h"
 #include "rclinit.h"
+#include "cstr.h"
 #include "mh_mbox.h"
 
 static char *thisprog;