diff --git a/arbor/include/arbor/util/expected.hpp b/arbor/include/arbor/util/expected.hpp
index be9dba141f17c20aeeb84f5d819fff0e5474eed4..17e37a80451947db697050b2d14ca500e413b57c 100644
--- a/arbor/include/arbor/util/expected.hpp
+++ b/arbor/include/arbor/util/expected.hpp
@@ -69,7 +69,7 @@ private:
 template <typename E>
 struct unexpected {
     template <typename F>
-    friend class unexpected;
+    friend struct unexpected;
 
     unexpected() = default;
     unexpected(const unexpected&) = default;