diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index 1366228..88a70e4 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -496,9 +496,11 @@ Resulting in these JSON objects:
 
 Notice that in a flat union, the discriminator name is controlled by
 the user, but because it must map to a base member with enum type, the
-code generator can ensure that branches exist for all values of the
-enum (although the order of the keys need not match the declaration of
-the enum).  In the resulting generated C data types, a flat union is
+code generator ensures that branches match the existing values of the
+enum. The order of the keys need not match the declaration of the enum.
+The keys need not cover all possible enum values. Omitted enum values
+are still valid branches that add no additional members to the data type.
+In the resulting generated C data types, a flat union is
 represented as a struct with the base members included directly, and
 then a union of structures for each branch of the struct.
 
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.json b/tests/qapi-schema/flat-union-incomplete-branch.json
deleted file mode 100644
index 25a411b..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.json
+++ /dev/null
@@ -1,9 +0,0 @@
-# we require all branches of the union to be covered
-{ 'enum': 'TestEnum',
-  'data': [ 'value1', 'value2' ] }
-{ 'struct': 'TestTypeA',
-  'data': { 'string': 'str' } }
-{ 'union': 'TestUnion',
-  'base': { 'type': 'TestEnum' },
-  'discriminator': 'type',
-  'data': { 'value1': 'TestTypeA' } }
diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
index 46c7282..7b59817 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -39,7 +39,7 @@
             '*enum1': 'EnumOne' } }   # intentional forward reference
 
 { 'enum': 'EnumOne',
-  'data': [ 'value1', 'value2', 'value3' ] }
+  'data': [ 'value1', 'value2', 'value3', 'value4' ] }
 
 { 'struct': 'UserDefZero',
   'data': { 'integer': 'int' } }
@@ -76,7 +76,9 @@
   'discriminator': 'enum1',
   'data': { 'value1' : 'UserDefA',
             'value2' : 'UserDefB',
-            'value3' : 'UserDefB' } }
+            'value3' : 'UserDefB'
+            # 'value4' defaults to empty
+  } }
 
 { 'struct': 'UserDefUnionBase',
   'base': 'UserDefZero',
diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
index 2462fc0..4b53f08 100644
--- a/scripts/qapi/common.py
+++ b/scripts/qapi/common.py
@@ -779,13 +779,6 @@ 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']
@@ -1357,6 +1350,14 @@ 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/scripts/qapi/types.py b/scripts/qapi/types.py
index 64d9c0f..a599352 100644
--- a/scripts/qapi/types.py
+++ b/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/scripts/qapi/visit.py b/scripts/qapi/visit.py
index 5d72d89..60ae8cf 100644
--- a/scripts/qapi/visit.py
+++ b/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:
diff --git a/tests/Makefile.include b/tests/Makefile.include
index ca91da2..114aab7 100644
--- a/tests/Makefile.include
+++ b/tests/Makefile.include
@@ -498,7 +498,6 @@ qapi-schema += flat-union-base-any.json
 qapi-schema += flat-union-base-union.json
 qapi-schema += flat-union-clash-member.json
 qapi-schema += flat-union-empty.json
-qapi-schema += flat-union-incomplete-branch.json
 qapi-schema += flat-union-inline.json
 qapi-schema += flat-union-int-branch.json
 qapi-schema += flat-union-invalid-branch-key.json
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.err b/tests/qapi-schema/flat-union-incomplete-branch.err
deleted file mode 100644
index e826bf0..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.err
+++ /dev/null
@@ -1 +0,0 @@
-tests/qapi-schema/flat-union-incomplete-branch.json:6: Union 'TestUnion' data missing 'value2' branch
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.exit b/tests/qapi-schema/flat-union-incomplete-branch.exit
deleted file mode 100644
index d00491f..0000000
--- a/tests/qapi-schema/flat-union-incomplete-branch.exit
+++ /dev/null
@@ -1 +0,0 @@
-1
diff --git a/tests/qapi-schema/flat-union-incomplete-branch.out b/tests/qapi-schema/flat-union-incomplete-branch.out
deleted file mode 100644
index e69de29..0000000
diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
index 542a19c..0dbcdaf 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -23,7 +23,7 @@ object UserDefOne
     base UserDefZero
     member string: str optional=False
     member enum1: EnumOne optional=True
-enum EnumOne ['value1', 'value2', 'value3']
+enum EnumOne ['value1', 'value2', 'value3', 'value4']
 object UserDefZero
     member integer: int optional=False
 object UserDefTwoDictDict
@@ -52,6 +52,7 @@ object UserDefFlatUnion
     case value1: UserDefA
     case value2: UserDefB
     case value3: UserDefB
+    case value4: q_empty
 object UserDefUnionBase
     base UserDefZero
     member string: str optional=False
