diff --git a/include/stringops.h b/include/stringops.h
index 9973485..4fca34f 100644
--- a/include/stringops.h
+++ b/include/stringops.h
@@ -2,6 +2,7 @@
 #define __STRINGOPS_H
 
 #include <silo.h>
+#include <stddef.h>
 
 /* common */
 #ifndef __SIZE_TYPE__