diff --git a/prick_vec.h b/prick_vec.h
index 8daafaa..0b88494 100644
--- a/prick_vec.h
+++ b/prick_vec.h
@@ -27,6 +27,7 @@
 #define PRICK_VEC_H
 
 #include <assert.h>
+#include <stddef.h>
 #include <stdint.h>
 
 #define PRICK_VEC_INLINE_CAPACITY 32