diff --git a/src/stack_allocator.c b/src/stack_allocator.c
index bd255b404..2b6b408c1 100644
--- a/src/stack_allocator.c
+++ b/src/stack_allocator.c
@@ -7,6 +7,7 @@
 #include "core.h"
 
 #include <stdbool.h>
+#include <stddef.h>
 
 typedef struct b2StackEntry
 {