diff --git a/include/pocketpy/cffi.h b/include/pocketpy/cffi.h
index b3cf5917..8fdb6dff 100644
--- a/include/pocketpy/cffi.h
+++ b/include/pocketpy/cffi.h
@@ -93,7 +93,7 @@ struct C99Struct{
     char* p;
     int size;
 
-    C99Struct(int new_size, bool zero_init){
+    C99Struct(int new_size, bool zero_init=true){
         this->size = new_size;
         if(size <= INLINE_SIZE){
             p = _inlined;