diff --git a/folly/futures/helpers.h b/folly/futures/helpers.h
index 7fd7950..c39d668 100644
--- a/folly/futures/helpers.h
+++ b/folly/futures/helpers.h
@@ -58,7 +58,8 @@ namespace futures {
    */
   template <class It, class F,
             class ItT = typename std::iterator_traits<It>::value_type,
-            class Result = decltype(std::declval<ItT>().then(std::declval<F>()))>
+            class Result
+      = typename decltype(std::declval<ItT>().then(std::declval<F>()))::value_type>
   std::vector<Future<Result>> map(It first, It last, F func);
 
   // Sugar for the most common case
diff --git a/folly/futures/test/FutureTest.cpp b/folly/futures/test/FutureTest.cpp
index da3e1c5..5b7be30 100644
--- a/folly/futures/test/FutureTest.cpp
+++ b/folly/futures/test/FutureTest.cpp
@@ -1765,7 +1765,7 @@ TEST(Map, Basic) {
   fs.push_back(p3.getFuture());
 
   int c = 0;
-  auto fs2 = futures::map(fs, [&](int i){
+  std::vector<Future<void>> fs2 = futures::map(fs, [&](int i){
     c += i;
   });