diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index 1967adfa92..c6dd1891c3 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -1446,6 +1446,8 @@ Example:
             goto out;
         }
         if (!*obj) {
+            /* incomplete */
+            assert(visit_is_dealloc(v));
             goto out_obj;
         }
         visit_type_UserDefOne_members(v, *obj, &err);
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index a425ea514c..2d40d2fe0f 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -479,6 +479,11 @@ void visit_type_enum(Visitor *v, const char *name, int *obj,
  */
 bool visit_is_input(Visitor *v);
 
+/*
+ * Check if visitor is a dealloc visitor.
+ */
+bool visit_is_dealloc(Visitor *v);
+
 /*** Visiting built-in types ***/
 
 /*
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 5365561b07..d4aac206cf 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -142,6 +142,11 @@ bool visit_is_input(Visitor *v)
     return v->type == VISITOR_INPUT;
 }
 
+bool visit_is_dealloc(Visitor *v)
+{
+    return v->type == VISITOR_DEALLOC;
+}
+
 void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error **errp)
 {
     assert(obj);
diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
index 23d9194aa4..e3467b770b 100644
--- a/scripts/qapi/visit.py
+++ b/scripts/qapi/visit.py
@@ -189,6 +189,8 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
         goto out;
     }
     if (!*obj) {
+        /* incomplete */
+        assert(visit_is_dealloc(v));
         goto out_obj;
     }
     switch ((*obj)->type) {
@@ -260,6 +262,8 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
         goto out;
     }
     if (!*obj) {
+        /* incomplete */
+        assert(visit_is_dealloc(v));
         goto out_obj;
     }
     visit_type_%(c_name)s_members(v, *obj, &err);