diff --git a/src/utils.hh b/src/utils.hh
index 7834837e..75129556 100644
--- a/src/utils.hh
+++ b/src/utils.hh
@@ -144,7 +144,7 @@ public:
 template <typename T> struct Vec {
   std::unique_ptr<T[]> a;
   int s = 0;
-#if !(__clang__ || __GNUC__ > 7 || __GNUC__ == 7 && __GNUC_MINOR__ >= 4)
+#if !(__clang__ || __GNUC__ > 7 || __GNUC__ == 7 && __GNUC_MINOR__ >= 4) || defined(_WIN32)
   // Work around a bug in GCC<7.4 that optional<IndexUpdate> would not be
   // construtible.
   Vec() = default;