diff --git a/qemu/scripts/qapi/common.py b/qemu/scripts/qapi/common.py
index 77e0be72..0e676f2a 100644
--- a/qemu/scripts/qapi/common.py
+++ b/qemu/scripts/qapi/common.py
@@ -791,13 +791,6 @@ def check_union(expr, info):
                                    "enum '%s'"
                                    % (key, enum_define['enum']))
 
-    # If discriminator is user-defined, ensure all values are covered
-    if enum_define:
-        for value in enum_define['data']:
-            if value not in members.keys():
-                raise QAPISemError(info, "Union '%s' data missing '%s' branch"
-                                   % (name, value))
-
 
 def check_alternate(expr, info):
     name = expr['alternate']
@@ -1367,6 +1360,14 @@ class QAPISchemaObjectTypeVariants(object):
             self.tag_member = seen[c_name(self._tag_name)]
             assert self._tag_name == self.tag_member.name
         assert isinstance(self.tag_member.type, QAPISchemaEnumType)
+        if self._tag_name:    # flat union
+            # branches that are not explicitly covered get an empty type
+            cases = set([v.name for v in self.variants])
+            for val in self.tag_member.type.values:
+                if val.name not in cases:
+                    v = QAPISchemaObjectTypeVariant(val.name, 'q_empty')
+                    v.set_owner(self.tag_member.owner)
+                    self.variants.append(v)
         for v in self.variants:
             v.check(schema)
             # Union names must match enum values; alternate names are
diff --git a/qemu/scripts/qapi/types.py b/qemu/scripts/qapi/types.py
index 839cd23b..37824510 100644
--- a/qemu/scripts/qapi/types.py
+++ b/qemu/scripts/qapi/types.py
@@ -125,6 +125,8 @@ def gen_variants(variants):
                 c_name=c_name(variants.tag_member.name))
 
     for var in variants.variants:
+        if var.type.name == 'q_empty':
+            continue
         ret += mcgen('''
         %(c_type)s %(c_name)s;
 ''',
diff --git a/qemu/scripts/qapi/visit.py b/qemu/scripts/qapi/visit.py
index 419a58bb..f267a7df 100644
--- a/qemu/scripts/qapi/visit.py
+++ b/qemu/scripts/qapi/visit.py
@@ -81,15 +81,24 @@ void visit_type_%(c_name)s_members(Visitor *v, %(c_name)s *obj, Error **errp)
                      c_name=c_name(variants.tag_member.name))
 
         for var in variants.variants:
-            ret += mcgen('''
+            case_str = c_enum_const(variants.tag_member.type.name,
+                                    var.name,
+                                    variants.tag_member.type.prefix)
+            if var.type.name == 'q_empty':
+                # valid variant and nothing to do
+                ret += mcgen('''
+    case %(case)s:
+        break;
+''',
+                             case=case_str)
+            else:
+                ret += mcgen('''
     case %(case)s:
         visit_type_%(c_type)s_members(v, &obj->u.%(c_name)s, &err);
         break;
 ''',
-                         case=c_enum_const(variants.tag_member.type.name,
-                                           var.name,
-                                           variants.tag_member.type.prefix),
-                         c_type=var.type.c_name(), c_name=c_name(var.name))
+                             case=case_str,
+                             c_type=var.type.c_name(), c_name=c_name(var.name))
 
         ret += mcgen('''
     default: