diff --git a/pykd/customtypes.cpp b/pykd/customtypes.cpp
index cf0af7f..0b1020a 100644
--- a/pykd/customtypes.cpp
+++ b/pykd/customtypes.cpp
@@ -8,25 +8,7 @@ namespace pykd {
 
 ////////////////////////////////////////////////////////////////////////////////
 
-TypeInfoPtr CustomStruct::create(const std::string &name, ULONG alignReq /*= 0*/)
-{
-    return TypeInfoPtr( new CustomStruct(name, alignReq) );
-}
-
-////////////////////////////////////////////////////////////////////////////////
-
-ULONG CustomStruct::getSize()
-{
-    if (UdtFieldColl::empty())
-        return 0;
-
-    UdtUtils::Field &field = UdtFieldColl::last();
-    return field.m_offset + field.m_type->getSize();
-}
-
-////////////////////////////////////////////////////////////////////////////////
-
-void CustomStruct::appendField(const std::string &fieldName, TypeInfoPtr fieldType)
+void CustomTypeBase::throwIfFiledExist(const std::string &fieldName)
 {
     bool fieldExist = false;
     try
@@ -40,10 +22,71 @@ void CustomStruct::appendField(const std::string &fieldName, TypeInfoPtr fieldTy
     }
     if (fieldExist)
         throw TypeException(getName(), "duplicate field name: " + fieldName);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+void CustomTypeBase::throwIfTypeRecursive(TypeInfoPtr type)
+{
+    if (type->is(this))
+        throw TypeException(getName(), "wrong type of field");
+
+    return throwIfTypeRecursiveImpl(type);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+void CustomTypeBase::throwIfTypeRecursiveImpl(TypeInfoPtr type)
+{
+    if (type->isEnum())
+        return;
+
+    try
+    {
+        const ULONG fields = type->getFieldCount();
+        for (ULONG i = 0; i < fields; ++i)
+        {
+            TypeInfoPtr fileldType = type->getFieldByIndex(i);
+            if (fileldType->is(this))
+                throw TypeException(getName(), "wrong type of field");
+
+            throwIfTypeRecursiveImpl(fileldType);
+        }
+    }
+    catch (const TypeException &except)
+    {
+        DBG_UNREFERENCED_PARAMETER(except);
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+TypeInfoPtr CustomStruct::create(const std::string &name, ULONG alignReq /*= 0*/)
+{
+    return TypeInfoPtr( new CustomStruct(name, alignReq) );
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+ULONG CustomStruct::getSize()
+{
+    if (Base::empty())
+        return 0;
+
+    UdtUtils::Field &field = Base::last();
+    return field.m_offset + field.m_type->getSize();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+void CustomStruct::appendField(const std::string &fieldName, TypeInfoPtr fieldType)
+{
+    Base::throwIfFiledExist(fieldName);
+    Base::throwIfTypeRecursive(fieldType);
 
     ULONG offset = getSize();
     offset += offset % (m_align ? m_align : fieldType->getAlignReq());
-    UdtFieldColl::push_back(
+    Base::push_back(
         UdtUtils::Field(offset, fieldName, fieldType)
     );
 }
@@ -73,20 +116,10 @@ ULONG CustomUnion::getSize()
 
 void CustomUnion::appendField(const std::string &fieldName, TypeInfoPtr fieldType)
 {
-    bool fieldExist = false;
-    try
-    {
-        lookupField(fieldName);
-        fieldExist = true;
-    }
-    catch (const TypeException &except)
-    {
-        DBG_UNREFERENCED_PARAMETER(except);
-    }
-    if (fieldExist)
-        throw TypeException(getName(), "duplicate field name: " + fieldName);
+    Base::throwIfFiledExist(fieldName);
+    Base::throwIfTypeRecursive(fieldType);
 
-    UdtFieldColl::push_back(
+    Base::push_back(
         UdtUtils::Field(0, fieldName, fieldType)
     );
 }
diff --git a/pykd/customtypes.h b/pykd/customtypes.h
index ddfd44e..32c0747 100644
--- a/pykd/customtypes.h
+++ b/pykd/customtypes.h
@@ -12,14 +12,30 @@ namespace pykd {
 
 ////////////////////////////////////////////////////////////////////////////////
 
-class CustomStruct : public UdtFieldColl
+class CustomTypeBase : public UdtFieldColl
 {
+    typedef UdtFieldColl Base;
+protected:
+    CustomTypeBase(const std::string &name) : UdtFieldColl(name) {}
+
+    void throwIfFiledExist(const std::string &fieldName);
+    void throwIfTypeRecursive(TypeInfoPtr type);
+
+private:
+    void throwIfTypeRecursiveImpl(TypeInfoPtr type);
+};
+
+////////////////////////////////////////////////////////////////////////////////
+
+class CustomStruct : public CustomTypeBase
+{
+    typedef CustomTypeBase Base;
 public:
     static TypeInfoPtr create(const std::string &name, ULONG align = 0);
 
 protected:
     CustomStruct(const std::string &name, ULONG align)
-        : UdtFieldColl(name), m_name(name), m_align(align) 
+        : Base(name), m_name(name), m_align(align) 
     {
     }
 
@@ -42,13 +58,14 @@ private:
 
 ////////////////////////////////////////////////////////////////////////////////
 
-class CustomUnion : public UdtFieldColl
+class CustomUnion : public CustomTypeBase
 {
+    typedef CustomTypeBase Base;
 public:
     static TypeInfoPtr create(const std::string &name);
 
 protected:
-    CustomUnion(const std::string &name) : UdtFieldColl(name) {}
+    CustomUnion(const std::string &name) : Base(name) {}
 
     virtual ULONG getSize() override;
 
diff --git a/pykd/typeinfo.h b/pykd/typeinfo.h
index 046fcff..fe033ef 100644
--- a/pykd/typeinfo.h
+++ b/pykd/typeinfo.h
@@ -211,6 +211,10 @@ public:
         return m_virtualBaseType;
     }
 
+    bool is(TypeInfo *rhs) const {
+        return this == rhs;
+    }
+
 protected:
 
     std::string getComplexName();