diff --git a/subprocess.hpp b/subprocess.hpp
index c4bcbba..d3840a5 100755
--- a/subprocess.hpp
+++ b/subprocess.hpp
@@ -932,7 +932,7 @@ private:
 class Popen
 {
 public:
-  friend class detail::ArgumentDeducer;
+  friend struct detail::ArgumentDeducer;
   friend class detail::Child;
 
   template <typename... Args>