diff --git a/tommath.h b/tommath.h
index ef7db3ae7..2af9072cb 100644
--- a/tommath.h
+++ b/tommath.h
@@ -5,6 +5,7 @@
 #define BN_H_
 
 #include <stdint.h>
+#include <stddef.h>
 #include <limits.h>
 
 #ifdef LTM_NO_FILE