diff --git a/src/pv/number.c b/src/pv/number.c
index d841402..409e468 100644
--- a/src/pv/number.c
+++ b/src/pv/number.c
@@ -2,6 +2,8 @@
  * Functions for converting strings to numbers.
  */
 
+#include <stddef.h>
+
 #ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif