diff --git a/include/qom/object.h b/include/qom/object.h
index f9ad692f21..bd68d1938c 100644
--- a/include/qom/object.h
+++ b/include/qom/object.h
@@ -200,8 +200,14 @@ typedef struct InterfaceInfo InterfaceInfo;
  *
  * Interfaces allow a limited form of multiple inheritance.  Instances are
  * similar to normal types except for the fact that are only defined by
- * their classes and never carry any state.  You can dynamically cast an object
- * to one of its #Interface types and vice versa.
+ * their classes and never carry any state.  As a consequence, a pointer to
+ * an interface instance should always be of incomplete type in order to be
+ * sure it cannot be dereferenced.  That is, you should define the
+ * 'typedef struct SomethingIf SomethingIf' so that you can pass around
+ * 'SomethingIf *si' arguments, but not define a 'struct SomethingIf { ... }'.
+ * The only things you can validly do with a 'SomethingIf *' are to pass it as
+ * an argument to a method on its corresponding SomethingIfClass, or to
+ * dynamically cast it to an object that implements the interface.
  *
  * # Methods #
  *