diff --git a/include/xo/facet/OObject.hpp b/include/xo/facet/OObject.hpp
index 7c87a66..1f46fad 100644
--- a/include/xo/facet/OObject.hpp
+++ b/include/xo/facet/OObject.hpp
@@ -72,6 +72,8 @@ namespace xo {
             /** OObject is truthy **/
             operator bool() const { return data_ != nullptr; }
 
+            // ----- iface() for variant fat pointer -----
+
             /** interface pointer for variant OObject instances.
              *  These instance support runtime polymorphism.
              **/
@@ -88,6 +90,18 @@ namespace xo {
                 return std::launder(&iface_);
             }
 
+            /** non-const verison. Technically all interface methods are const.
+             *  But counterintuitive to have to mark interface methods const
+             *  that are dedicated to mutable data.
+             **/
+            FacetType * iface()
+                requires std::is_same_v<DataType, DVariantPlaceholder>
+            {
+                return std::launder(&iface_);
+            }
+
+            // ----- iface() for typed fat pointer -----
+
             /** interface pointer for OObject instance with representation
              *  known at compile time.
              *
@@ -102,6 +116,16 @@ namespace xo {
                 return &iface_;
             }
 
+            /** non-const verison. Technically all interface methods are const.
+             *  But counterintuitive to have to mark interface methods const
+             *  that are dedicated to mutable data.
+             **/
+            FacetType * iface()
+                requires(!std::is_same_v<DataType, DVariantPlaceholder>)
+            {
+                return &iface_;
+            }
+
             DataPtr data() const { return data_; }
 
             void reset() { data_ = nullptr; }
diff --git a/include/xo/facet/RRouter.hpp b/include/xo/facet/RRouter.hpp
index daa7887..56e521d 100644
--- a/include/xo/facet/RRouter.hpp
+++ b/include/xo/facet/RRouter.hpp
@@ -12,6 +12,10 @@ namespace xo {
         template <typename RRouter>
         consteval bool valid_object_router()
         {
+            static_assert(sizeof(RRouter) >= sizeof(RRouter::ObjectType),
+                              "Router type must inherit Router::ObjectType");
+            static_assert(std::is_convertible_v<RRouter, typename RRouter::ObjectType>,
+                              "Router type must inherit Router::ObjectType");
             static_assert(requires { typename RRouter::ObjectType; },
                               "Router type must provide typename Router::ObjectType");
             static_assert(valid_object_traits<RRouter::ObjectType>,