--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -3463,19 +3463,18 @@ while (true) {
             constructHolder = None
 
         if defaultValue and not isinstance(defaultValue, IDLNullValue):
             tag = defaultValue.type.tag()
 
             if tag in numericSuffixes or tag is IDLType.Tags.bool:
                 defaultStr = getHandleDefault(defaultValue)
                 value = declLoc + (".Value()" if nullable else "")
-                default = CGGeneric("%s.SetAs%s() = %s;" % (value,
-                                                            defaultValue.type,
-                                                            defaultStr))
+                default = CGGeneric("%s.RawSetAs%s() = %s;" %
+                                    (value, defaultValue.type, defaultStr))
             else:
                 default = CGGeneric(
                     handleDefaultStringValue(
                         defaultValue, "%s.SetStringData" % unionArgumentObj) +
                     ";")
 
             templateBody = CGIfElseWrapper("!(${haveValue})", default, templateBody)
 
@@ -3496,17 +3495,17 @@ while (true) {
             assert type.hasDictionaryType
             assert defaultValue.type.isDictionary()
             if not isMember and typeNeedsRooting(defaultValue.type):
                 ctorArgs = "cx"
             else:
                 ctorArgs = ""
             initDictionaryWithNull = CGIfWrapper(
                 CGGeneric("return false;"),
-                ('!%s.SetAs%s(%s).Init(cx, JS::NullHandleValue, "Member of %s")'
+                ('!%s.RawSetAs%s(%s).Init(cx, JS::NullHandleValue, "Member of %s")'
                  % (declLoc, getUnionMemberName(defaultValue.type),
                     ctorArgs, type)))
             templateBody = CGIfElseWrapper("!(${haveValue})",
                                            initDictionaryWithNull,
                                            templateBody)
 
         templateBody = CGList([constructDecl, templateBody], "\n")
 
@@ -7062,17 +7061,17 @@ def getUnionTypeTemplateVars(unionType, 
                 "mutableVal": "pvalue",
                 "declName": "memberSlot",
                 "holderName": "m" + name + "Holder",
                 }
             )
         jsConversion = CGWrapper(CGIndenter(CGGeneric(jsConversion)),
                                  pre=("tryNext = false;\n"
                                       "{ // scope for memberSlot\n"
-                                      "  %s& memberSlot = SetAs%s(%s);\n"
+                                      "  %s& memberSlot = RawSetAs%s(%s);\n"
                                       % (structType, name, ctorArgs)),
                                  post=("\n"
                                        "}\n"
                                       "return true;"))
         setter = ClassMethod("TrySetTo" + name, "bool",
                               [Argument("JSContext*", "cx"),
                                Argument("JS::Handle<JS::Value>", "value"),
                                Argument("JS::MutableHandle<JS::Value>", "pvalue"),
@@ -7127,54 +7126,65 @@ class CGUnionStruct(CGThing):
         traceCases = []
         unionValues = []
         if self.type.hasNullableType:
             enumValues.append("eNull")
             methods.append(ClassMethod("IsNull", "bool", [], const=True, inline=True,
                                        body="return mType == eNull;",
                                        bodyInHeader=True))
             methods.append(ClassMethod("SetNull", "void", [], inline=True,
-                                       body=("MOZ_ASSERT(mType == eUninitialized);\n"
+                                       body=("Uninit();\n"
                                              "mType = eNull;"),
                                        bodyInHeader=True))
             destructorCases.append(CGCase("eNull", None))
             assignmentCases.append(CGCase("eNull",
                                           CGGeneric("MOZ_ASSERT(mType == eUninitialized);\n"
                                                     "mType = eNull;")))
             toJSValCases.append(CGCase("eNull", CGGeneric("rval.setNull();\n"
                                                           "return true;")))
 
+        hasObjectType = any(t.isObject() for t in self.type.flatMemberTypes)
         for t in self.type.flatMemberTypes:
             vars = getUnionTypeTemplateVars(self.type,
                                             t, self.descriptorProvider,
                                             ownsMembers=self.ownsMembers)
             if vars["name"] != "Object" or self.ownsMembers:
                 body=string.Template("if (mType == e${name}) {\n"
                                      "  return mValue.m${name}.Value();\n"
                                      "}\n"
-                                     "MOZ_ASSERT(mType == eUninitialized);\n"
+                                     "%s\n"
                                      "mType = e${name};\n"
                                      "return mValue.m${name}.SetValue(${ctorArgs});").substitute(vars)
                 # bodyInHeader must be false for return values because they own
                 # their union members and we don't want include headers in
                 # UnionTypes.h just to call Addref/Release
-                methods.append(ClassMethod("SetAs" + vars["name"],
-                                           vars["structType"] + "&",
-                                           vars["ctorArgList"],
-                                           bodyInHeader=not self.ownsMembers,
-                                           body=body))
+                methods.append(ClassMethod(
+                    "RawSetAs" + vars["name"],
+                    vars["structType"] + "&",
+                    vars["ctorArgList"],
+                    bodyInHeader=not self.ownsMembers,
+                    body=body % "MOZ_ASSERT(mType == eUninitialized);"))
+                uninit = "Uninit();"
+                if hasObjectType and not self.ownsMembers:
+                    uninit = 'MOZ_ASSERT(mType != eObject, "This will not play well with Rooted");\n' + uninit
+                methods.append(ClassMethod(
+                    "SetAs" + vars["name"],
+                    vars["structType"] + "&",
+                    vars["ctorArgList"],
+                    bodyInHeader=not self.ownsMembers,
+                    body=body % uninit))
                 if self.ownsMembers:
                     methods.append(vars["setter"])
                     if t.isString():
                         methods.append(
                             ClassMethod("SetStringData", "void",
                                 [Argument("const nsString::char_type*", "aData"),
                                  Argument("nsString::size_type", "aLength")],
                                 inline=True, bodyInHeader=True,
-                                body="SetAsString().Assign(aData, aLength);"))
+                                body="RawSetAsString().Assign(aData, aLength);"))
 
             body = string.Template('MOZ_ASSERT(Is${name}(), "Wrong type!");\n'
                                    'mValue.m${name}.Destroy();\n'
                                    'mType = eUninitialized;').substitute(vars)
             methods.append(ClassMethod("Destroy" + vars["name"],
                                        "void",
                                        [],
                                        visibility="private",
@@ -7207,17 +7217,17 @@ class CGUnionStruct(CGThing):
 
             toJSValCases.append(CGCase("e" + vars["name"],
                                        self.getConversionToJS(vars, t)))
             destructorCases.append(CGCase("e" + vars["name"],
                                           CGGeneric("Destroy%s();"
                                                      % vars["name"])))
             assignmentCases.append(
                 CGCase("e" + vars["name"],
-                       CGGeneric("SetAs%s() = aOther.GetAs%s();" %
+                       CGGeneric("RawSetAs%s() = aOther.GetAs%s();" %
                                  (vars["name"], vars["name"]))))
             if self.ownsMembers and typeNeedsRooting(t):
                 if t.isObject():
                     traceCases.append(
                         CGCase("e" + vars["name"],
                                CGGeneric('JS_CallObjectTracer(trc, %s, "%s");' %
                                          ("&mValue.m" + vars["name"] + ".Value()",
                                           "mValue.m" + vars["name"]))))
@@ -7359,28 +7369,28 @@ class CGUnionConversionStruct(CGThing):
         for t in self.type.flatMemberTypes:
             vars = getUnionTypeTemplateVars(self.type,
                                             t, self.descriptorProvider)
             methods.append(vars["setter"])
             if vars["name"] != "Object":
                 body=string.Template("MOZ_ASSERT(mUnion.mType == mUnion.eUninitialized);\n"
                                      "mUnion.mType = mUnion.e${name};\n"
                                      "return mUnion.mValue.m${name}.SetValue(${ctorArgs});").substitute(vars)
-                methods.append(ClassMethod("SetAs" + vars["name"],
+                methods.append(ClassMethod("RawSetAs" + vars["name"],
                                            vars["structType"] + "&",
                                            vars["ctorArgList"],
                                            bodyInHeader=True,
                                            body=body,
                                            visibility="private"))
                 if t.isString():
                     methods.append(ClassMethod("SetStringData", "void",
                                      [Argument("const nsDependentString::char_type*", "aData"),
                                       Argument("nsDependentString::size_type", "aLength")],
                                      inline=True, bodyInHeader=True,
-                                     body="SetAsString().SetData(aData, aLength);"))
+                                     body="RawSetAsString().SetData(aData, aLength);"))
 
             if vars["holderType"] is not None:
                 members.append(ClassMember("m%sHolder" % vars["name"],
                                            vars["holderType"]))
 
         return CGClass(structName + "Argument",
                        members=members,
                        constructors=[ctor],