diff --git a/scripts/qapi.py b/scripts/qapi.py
index 1954292..cea346f 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -50,6 +50,15 @@ class QAPISchemaError(Exception):
     def __str__(self):
         return "%s:%s:%s: %s" % (self.fp.name, self.line, self.col, self.msg)
 
+class QAPIExprError(Exception):
+    def __init__(self, expr_info, msg):
+        self.fp = expr_info['fp']
+        self.line = expr_info['line']
+        self.msg = msg
+
+    def __str__(self):
+        return "%s:%s: %s" % (self.fp.name, self.line, self.msg)
+
 class QAPISchema:
 
     def __init__(self, fp):
@@ -64,7 +73,10 @@ class QAPISchema:
         self.accept()
 
         while self.tok != None:
-            self.exprs.append(self.get_expr(False))
+            expr_info = {'fp': fp, 'line': self.line}
+            expr_elem = {'expr': self.get_expr(False),
+                         'info': expr_info}
+            self.exprs.append(expr_elem)
 
     def accept(self):
         while True:
@@ -162,6 +174,89 @@ class QAPISchema:
             raise QAPISchemaError(self, 'Expected "{", "[" or string')
         return expr
 
+def find_base_fields(base):
+    base_struct_define = find_struct(base)
+    if not base_struct_define:
+        return None
+    return base_struct_define['data']
+
+# Return the discriminator enum define if discrminator is specified as an
+# enum type, otherwise return None.
+def discriminator_find_enum_define(expr):
+    base = expr.get('base')
+    discriminator = expr.get('discriminator')
+
+    if not (discriminator and base):
+        return None
+
+    base_fields = find_base_fields(base)
+    if not base_fields:
+        return None
+
+    discriminator_type = base_fields.get(discriminator)
+    if not discriminator_type:
+        return None
+
+    return find_enum(discriminator_type)
+
+def check_union(expr, expr_info):
+    name = expr['union']
+    base = expr.get('base')
+    discriminator = expr.get('discriminator')
+    members = expr['data']
+
+    # If the object has a member 'base', its value must name a complex type.
+    if base:
+        base_fields = find_base_fields(base)
+        if not base_fields:
+            raise QAPIExprError(expr_info,
+                                "Base '%s' is not a valid type"
+                                % base)
+
+    # If the union object has no member 'discriminator', it's an
+    # ordinary union.
+    if not discriminator:
+        enum_define = None
+
+    # Else if the value of member 'discriminator' is {}, it's an
+    # anonymous union.
+    elif discriminator == {}:
+        enum_define = None
+
+    # Else, it's a flat union.
+    else:
+        # The object must have a member 'base'.
+        if not base:
+            raise QAPIExprError(expr_info,
+                                "Flat union '%s' must have a base field"
+                                % name)
+        # The value of member 'discriminator' must name a member of the
+        # base type.
+        if not base_fields.get(discriminator):
+            raise QAPIExprError(expr_info,
+                                "Discriminator '%s' is not a member of base "
+                                "type '%s'"
+                                % (discriminator, base))
+        enum_define = discriminator_find_enum_define(expr)
+
+    # Check every branch
+    for (key, value) in members.items():
+        # If this named member's value names an enum type, then all members
+        # of 'data' must also be members of the enum type.
+        if enum_define and not key in enum_define['enum_values']:
+            raise QAPIExprError(expr_info,
+                                "Discriminator value '%s' is not found in "
+                                "enum '%s'" %
+                                (key, enum_define["enum_name"]))
+        # Todo: put more check such as whether each value is valid, but it may
+        # need more functions to handle array case, so leave it now.
+
+def check_exprs(schema):
+    for expr_elem in schema.exprs:
+        expr = expr_elem['expr']
+        if expr.has_key('union'):
+            check_union(expr, expr_elem['info'])
+
 def parse_schema(fp):
     try:
         schema = QAPISchema(fp)
@@ -171,7 +266,8 @@ def parse_schema(fp):
 
     exprs = []
 
-    for expr in schema.exprs:
+    for expr_elem in schema.exprs:
+        expr = expr_elem['expr']
         if expr.has_key('enum'):
             add_enum(expr['enum'], expr['data'])
         elif expr.has_key('union'):
@@ -181,6 +277,12 @@ def parse_schema(fp):
             add_struct(expr)
         exprs.append(expr)
 
+    try:
+        check_exprs(schema)
+    except QAPIExprError, e:
+        print >>sys.stderr, e
+        exit(1)
+
     return exprs
 
 def parse_args(typeinfo):
diff --git a/tests/Makefile b/tests/Makefile
index dfe06eb..6ac9889 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -143,7 +143,9 @@ check-qapi-schema-y := $(addprefix tests/qapi-schema/, \
         qapi-schema-test.json quoted-structural-chars.json \
         trailing-comma-list.json trailing-comma-object.json \
         unclosed-list.json unclosed-object.json unclosed-string.json \
-        duplicate-key.json)
+        duplicate-key.json union-invalid-base.json flat-union-no-base.json \
+        flat-union-invalid-discriminator.json \
+        flat-union-invalid-branch-key.json)
 
 GENERATED_HEADERS += tests/test-qapi-types.h tests/test-qapi-visit.h tests/test-qmp-commands.h
 
diff --git a/tests/qapi-schema/flat-union-invalid-branch-key.err b/tests/qapi-schema/flat-union-invalid-branch-key.err
new file mode 100644
index 0000000..1125caf
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-branch-key.err
@@ -0,0 +1 @@
+<stdin>:13: Discriminator value 'value_wrong' is not found in enum 'TestEnum'
diff --git a/tests/qapi-schema/flat-union-invalid-branch-key.exit b/tests/qapi-schema/flat-union-invalid-branch-key.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-branch-key.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/flat-union-invalid-branch-key.json b/tests/qapi-schema/flat-union-invalid-branch-key.json
new file mode 100644
index 0000000..a624282
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-branch-key.json
@@ -0,0 +1,17 @@
+{ 'enum': 'TestEnum',
+  'data': [ 'value1', 'value2' ] }
+
+{ 'type': 'TestBase',
+  'data': { 'enum1': 'TestEnum' } }
+
+{ 'type': 'TestTypeA',
+  'data': { 'string': 'str' } }
+
+{ 'type': 'TestTypeB',
+  'data': { 'integer': 'int' } }
+
+{ 'union': 'TestUnion',
+  'base': 'TestBase',
+  'discriminator': 'enum1',
+  'data': { 'value_wrong': 'TestTypeA',
+            'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/flat-union-invalid-branch-key.out b/tests/qapi-schema/flat-union-invalid-branch-key.out
new file mode 100644
index 0000000..e69de29
diff --git a/tests/qapi-schema/flat-union-invalid-discriminator.err b/tests/qapi-schema/flat-union-invalid-discriminator.err
new file mode 100644
index 0000000..cad9dbf
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-discriminator.err
@@ -0,0 +1 @@
+<stdin>:13: Discriminator 'enum_wrong' is not a member of base type 'TestBase'
diff --git a/tests/qapi-schema/flat-union-invalid-discriminator.exit b/tests/qapi-schema/flat-union-invalid-discriminator.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-discriminator.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/flat-union-invalid-discriminator.json b/tests/qapi-schema/flat-union-invalid-discriminator.json
new file mode 100644
index 0000000..887157e
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-discriminator.json
@@ -0,0 +1,17 @@
+{ 'enum': 'TestEnum',
+  'data': [ 'value1', 'value2' ] }
+
+{ 'type': 'TestBase',
+  'data': { 'enum1': 'TestEnum' } }
+
+{ 'type': 'TestTypeA',
+  'data': { 'string': 'str' } }
+
+{ 'type': 'TestTypeB',
+  'data': { 'integer': 'int' } }
+
+{ 'union': 'TestUnion',
+  'base': 'TestBase',
+  'discriminator': 'enum_wrong',
+  'data': { 'value1': 'TestTypeA',
+            'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/flat-union-invalid-discriminator.out b/tests/qapi-schema/flat-union-invalid-discriminator.out
new file mode 100644
index 0000000..e69de29
diff --git a/tests/qapi-schema/flat-union-no-base.err b/tests/qapi-schema/flat-union-no-base.err
new file mode 100644
index 0000000..e695315
--- /dev/null
+++ b/tests/qapi-schema/flat-union-no-base.err
@@ -0,0 +1 @@
+<stdin>:13: Flat union 'TestUnion' must have a base field
diff --git a/tests/qapi-schema/flat-union-no-base.exit b/tests/qapi-schema/flat-union-no-base.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/flat-union-no-base.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/flat-union-no-base.json b/tests/qapi-schema/flat-union-no-base.json
new file mode 100644
index 0000000..e0900d4
--- /dev/null
+++ b/tests/qapi-schema/flat-union-no-base.json
@@ -0,0 +1,16 @@
+{ 'enum': 'TestEnum',
+  'data': [ 'value1', 'value2' ] }
+
+{ 'type': 'TestBase',
+  'data': { 'enum1': 'TestEnum' } }
+
+{ 'type': 'TestTypeA',
+  'data': { 'string': 'str' } }
+
+{ 'type': 'TestTypeB',
+  'data': { 'integer': 'int' } }
+
+{ 'union': 'TestUnion',
+  'discriminator': 'enum1',
+  'data': { 'value1': 'TestTypeA',
+            'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/flat-union-no-base.out b/tests/qapi-schema/flat-union-no-base.out
new file mode 100644
index 0000000..e69de29
diff --git a/tests/qapi-schema/union-invalid-base.err b/tests/qapi-schema/union-invalid-base.err
new file mode 100644
index 0000000..dd8e3d1
--- /dev/null
+++ b/tests/qapi-schema/union-invalid-base.err
@@ -0,0 +1 @@
+<stdin>:7: Base 'TestBaseWrong' is not a valid type
diff --git a/tests/qapi-schema/union-invalid-base.exit b/tests/qapi-schema/union-invalid-base.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/union-invalid-base.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/union-invalid-base.json b/tests/qapi-schema/union-invalid-base.json
new file mode 100644
index 0000000..1fa4930
--- /dev/null
+++ b/tests/qapi-schema/union-invalid-base.json
@@ -0,0 +1,10 @@
+{ 'type': 'TestTypeA',
+  'data': { 'string': 'str' } }
+
+{ 'type': 'TestTypeB',
+  'data': { 'integer': 'int' } }
+
+{ 'union': 'TestUnion',
+  'base': 'TestBaseWrong',
+  'data': { 'value1': 'TestTypeA',
+            'value2': 'TestTypeB' } }
