diff --git a/uno/tools/UserCallbacks.hpp b/uno/tools/UserCallbacks.hpp
index 97e39e46..0939c73c 100644
--- a/uno/tools/UserCallbacks.hpp
+++ b/uno/tools/UserCallbacks.hpp
@@ -6,7 +6,7 @@
 
 namespace uno {
    // forward declarations
-   class Multipliers;
+   struct Multipliers;
    template <class ElementType>
    class Vector;