diff --git a/include/mruby.h b/include/mruby.h
index 3ed86f8c7..4048edf69 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -162,11 +162,11 @@ struct mrb_state;
  *
  * The function pointing it must behave similarly as realloc except:
  * - If ptr is NULL it must allocate new space.
- * - If s is NULL, ptr must be freed.
+ * - If size is zero, ptr must be freed.
  *
  * See @see mrb_default_allocf for the default implementation.
  */
-typedef void* (*mrb_allocf) (struct mrb_state *mrb, void*, size_t, void *ud);
+typedef void* (*mrb_allocf) (struct mrb_state *mrb, void *ptr, size_t size, void *ud);
 
 #ifndef MRB_FIXED_STATE_ATEXIT_STACK_SIZE
 #define MRB_FIXED_STATE_ATEXIT_STACK_SIZE 5