diff --git a/scripts/qapi.py b/scripts/qapi.py
index 79038a8..b068141 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -977,20 +977,23 @@ class QAPISchemaObjectType(QAPISchemaType):
             self.base = schema.lookup_type(self._base_name)
             assert isinstance(self.base, QAPISchemaObjectType)
             self.base.check(schema)
-            self.base.check_clash(schema, seen)
+            self.base.check_clash(schema, self.info, seen)
         for m in self.local_members:
             m.check(schema)
-            m.check_clash(seen)
+            m.check_clash(self.info, seen)
         self.members = seen.values()
         if self.variants:
             self.variants.check(schema, seen)
             assert self.variants.tag_member in self.members
-            self.variants.check_clash(schema, seen)
+            self.variants.check_clash(schema, self.info, seen)

-    def check_clash(self, schema, seen):
+    # Check that the members of this type do not cause duplicate JSON fields,
+    # and update seen to track the members seen so far. Report any errors
+    # on behalf of info, which is not necessarily self.info
+    def check_clash(self, schema, info, seen):
         assert not self.variants       # not implemented
         for m in self.members:
-            m.check_clash(seen)
+            m.check_clash(info, seen)

     def is_implicit(self):
         # See QAPISchema._make_implicit_object_type()
@@ -1036,10 +1039,13 @@ class QAPISchemaObjectTypeMember(object):
         self.type = schema.lookup_type(self._type_name)
         assert self.type

-    def check_clash(self, seen):
-        # TODO change key of seen from QAPI name to C name
-        assert self.name not in seen
-        seen[self.name] = self
+    def check_clash(self, info, seen):
+        cname = c_name(self.name)
+        if cname in seen:
+            raise QAPIExprError(info,
+                                "%s collides with %s"
+                                % (self.describe(), seen[cname].describe()))
+        seen[cname] = self

     def _pretty_owner(self):
         owner = self.owner
@@ -1079,7 +1085,8 @@ class QAPISchemaObjectTypeVariants(object):

     def check(self, schema, seen):
         if not self.tag_member:    # flat union
-            self.tag_member = seen[self.tag_name]
+            self.tag_member = seen[c_name(self.tag_name)]
+            assert self.tag_name == self.tag_member.name
         assert isinstance(self.tag_member.type, QAPISchemaEnumType)
         for v in self.variants:
             v.check(schema)
@@ -1087,12 +1094,12 @@ class QAPISchemaObjectTypeVariants(object):
             if isinstance(v.type, QAPISchemaObjectType):
                 v.type.check(schema)

-    def check_clash(self, schema, seen):
+    def check_clash(self, schema, info, seen):
         for v in self.variants:
             # Reset seen map for each variant, since qapi names from one
             # branch do not affect another branch
             assert isinstance(v.type, QAPISchemaObjectType)
-            v.type.check_clash(schema, dict(seen))
+            v.type.check_clash(schema, info, dict(seen))


 class QAPISchemaObjectTypeVariant(QAPISchemaObjectTypeMember):
diff --git a/tests/qapi-schema/args-name-clash.err b/tests/qapi-schema/args-name-clash.err
index e69de29..d953e8d 100644
--- a/tests/qapi-schema/args-name-clash.err
+++ b/tests/qapi-schema/args-name-clash.err
@@ -0,0 +1 @@
+tests/qapi-schema/args-name-clash.json:4: 'a_b' (parameter of oops) collides with 'a-b' (parameter of oops)
diff --git a/tests/qapi-schema/args-name-clash.exit b/tests/qapi-schema/args-name-clash.exit
index 573541a..d00491f 100644
--- a/tests/qapi-schema/args-name-clash.exit
+++ b/tests/qapi-schema/args-name-clash.exit
@@ -1 +1 @@
-0
+1
diff --git a/tests/qapi-schema/args-name-clash.json b/tests/qapi-schema/args-name-clash.json
index 9e8f889..61423cb 100644
--- a/tests/qapi-schema/args-name-clash.json
+++ b/tests/qapi-schema/args-name-clash.json
@@ -1,5 +1,4 @@
 # C member name collision
-# FIXME - This parses, but fails to compile, because the C struct is given
-# two 'a_b' members.  Either reject this at parse time, or munge the C names
-# to avoid the collision.
+# Reject members that clash when mapped to C names (we would have two 'a_b'
+# members).
 { 'command': 'oops', 'data': { 'a-b': 'str', 'a_b': 'str' } }
diff --git a/tests/qapi-schema/args-name-clash.out b/tests/qapi-schema/args-name-clash.out
index 9b2f6e4..e69de29 100644
--- a/tests/qapi-schema/args-name-clash.out
+++ b/tests/qapi-schema/args-name-clash.out
@@ -1,6 +0,0 @@
-object :empty
-object :obj-oops-arg
-    member a-b: str optional=False
-    member a_b: str optional=False
-command oops :obj-oops-arg -> None
-   gen=True success_response=True
