diff --git a/folly/wangle/rx/Observable.h b/folly/wangle/rx/Observable.h
index 98f6b43..b3f2556 100644
--- a/folly/wangle/rx/Observable.h
+++ b/folly/wangle/rx/Observable.h
@@ -16,9 +16,9 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Subject.h>
 #include <folly/wangle/rx/Subscription.h>
-#include <folly/wangle/rx/types.h>
 
 #include <folly/RWSpinLock.h>
 #include <folly/SmallLocks.h>
diff --git a/folly/wangle/rx/Observer.h b/folly/wangle/rx/Observer.h
index 3ce94ac..e82d4e6 100644
--- a/folly/wangle/rx/Observer.h
+++ b/folly/wangle/rx/Observer.h
@@ -16,7 +16,7 @@
 
 #pragma once
 
-#include <folly/wangle/rx/types.h>
+#include <folly/wangle/rx/types.h> // must come first
 #include <functional>
 #include <memory>
 #include <stdexcept>
diff --git a/folly/wangle/rx/Subject.h b/folly/wangle/rx/Subject.h
index ba66f86..c806d70 100644
--- a/folly/wangle/rx/Subject.h
+++ b/folly/wangle/rx/Subject.h
@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Observable.h>
 #include <folly/wangle/rx/Observer.h>
 
diff --git a/folly/wangle/rx/Subscription.h b/folly/wangle/rx/Subscription.h
index e017f4e..8445ccf 100644
--- a/folly/wangle/rx/Subscription.h
+++ b/folly/wangle/rx/Subscription.h
@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Observable.h>
 
 namespace folly { namespace wangle {