diff --git a/src/interop/interop_wrapper.cxx b/src/interop/interop_wrapper.cxx
index d26697c8..699d3b39 100644
--- a/src/interop/interop_wrapper.cxx
+++ b/src/interop/interop_wrapper.cxx
@@ -1196,10 +1196,15 @@ bool interop::GetSmartPtrInfo(TCppScope_t klass, TCppScope_t* raw,
 
 // type offsets --------------------------------------------------------------
 ptrdiff_t interop::GetBaseOffset(TCppScope_t derived, TCppScope_t base,
-                                 TCppObject_t /*address*/, int direction,
+                                 TCppObject_t address, int direction,
                                  bool rerror) {
   std::lock_guard<std::recursive_mutex> Lock(InterOpMutex);
-  intptr_t offset = Cpp::GetBaseClassOffset(derived, base);
+
+  // Up-casts may start from a subobject whose complete type is unknown. For
+  // down-casts, callers obtain the complete type with GetActualClass, so use
+  // its static layout; address points to the base rather than the derived type.
+  intptr_t offset = Cpp::GetBaseClassOffset(
+      derived, base, direction > 0 ? address.data : nullptr);
 
   if (offset == -1) // Cling error, treat silently
     return rerror ? (ptrdiff_t)offset : 0;
diff --git a/test/test_advancedcpp.py b/test/test_advancedcpp.py
index 39f6ab70..6bd7aa7b 100644
--- a/test/test_advancedcpp.py
+++ b/test/test_advancedcpp.py
@@ -1030,3 +1030,71 @@ def __cast_cpp__(self):
 
         for norm in [ns.norm_cr, ns.norm_m, ns.norm_v]:
             assert round(norm(p3) - pynorm, 8) == 0
+
+    def test30_virtual_base_of_hidden_type(self):
+        """Offset to a virtual base taken from the actual object"""
+
+        import cppjit
+
+        cppjit.cppdef("""\
+        namespace vbase_hidden {
+        struct IB { virtual ~IB() {} virtual long id() const { return 42; } };
+        struct ID1 : virtual IB {};
+        struct Lead { virtual ~Lead() {} long padding[2] = {}; };
+        struct ID2 : Lead, virtual IB {};
+        struct IC { virtual ~IC() {} long z = 7; };
+        struct M : Lead, IC {};
+        struct ID3 : virtual M {};
+        long call(IB* p) { return p->id(); }
+        long call_c(IC* p) { return p->z; }
+        }
+        namespace {
+        struct Other { virtual ~Other() {} long x[100] = {}; };
+        struct Hidden : Other, vbase_hidden::ID1, vbase_hidden::ID2 {};
+        struct Hidden3 : Other, vbase_hidden::ID1, vbase_hidden::ID3 { long y = 3; };
+        }
+        namespace vbase_hidden {
+        void* make2() { return static_cast<ID2*>(new Hidden); }
+        void* make3() { return static_cast<ID3*>(new Hidden3); }
+        }""")
+
+        ns = cppjit.gbl.vbase_hidden
+
+        d2 = cppjit.bind_object(ns.make2(), ns.ID2)
+        assert ns.call(d2) == 42
+        assert d2.id() == 42
+
+        d3 = cppjit.bind_object(ns.make3(), ns.ID3)
+        assert ns.call_c(d3) == 7
+        assert d3.z == 7
+
+    def test31_virtual_base_downcast(self):
+        """GetActualClass supplies the complete type for virtual-base downcasts"""
+
+        import cppjit
+
+        cppjit.cppdef("""\
+        namespace vbase_downcast {
+        struct IB { virtual ~IB() {} virtual long id() const { return 42; } };
+        struct ID1 : virtual IB {};
+        struct ID2 : virtual IB {};
+        struct IC { virtual ~IC() {} long z = 7; };
+        struct Pad { virtual ~Pad() {} long padding[2] = {}; };
+        struct M : Pad, IC {};
+        struct ID3 : virtual M {};
+        struct Visible : ID1, ID2 { long value = 19; };
+        struct Visible3 : ID1, ID3 { long value = 23; };
+        IB* make() { return new Visible; }
+        IC* make3() { return new Visible3; }
+        }""")
+
+        ns = cppjit.gbl.vbase_downcast
+        obj = ns.make()
+        assert isinstance(obj, ns.Visible)
+        assert obj.value == 19
+        assert obj.id() == 42
+
+        obj3 = ns.make3()
+        assert isinstance(obj3, ns.Visible3)
+        assert obj3.value == 23
+        assert obj3.z == 7