diff --git a/dlib/type_safe_union/type_safe_union_kernel_abstract.h b/dlib/type_safe_union/type_safe_union_kernel_abstract.h
index bcbb45e65..91c42f6e9 100644
--- a/dlib/type_safe_union/type_safe_union_kernel_abstract.h
+++ b/dlib/type_safe_union/type_safe_union_kernel_abstract.h
@@ -167,7 +167,7 @@ namespace dlib
         >
     void serialize (
         const type_safe_union<T1,T2,T3,T4,T5,T6,T7,T8,T9,T10>& item, 
-        std::istream& in
+        std::ostream& out 
     );   
     /*!
         provides serialization support