diff --git a/src/closure.h b/src/closure.h
index fa02dd8..b044dc6 100644
--- a/src/closure.h
+++ b/src/closure.h
@@ -44,7 +44,7 @@ void
 closure_destroy(closure_t *closure) {
   if (closure->len) {
     free(closure->data);
-    V8::AdjustAmountOfExternalAllocatedMemory(-closure->max_len);
+    V8::AdjustAmountOfExternalAllocatedMemory(- (intptr_t) closure->max_len);
   }
 }