--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -7230,16 +7230,21 @@ class CGUnionStruct(CGThing):
                     assert t.isSpiderMonkeyInterface()
                     traceCases.append(
                         CGCase("e" + vars["name"],
                                CGGeneric("mValue.m%s.Value().TraceSelf(trc);" %
                                          vars["name"])))
 
         dtor = CGSwitch("mType", destructorCases).define()
 
+        methods.append(ClassMethod("Uninit", "void", [],
+                                   visibility="private", body=dtor,
+                                   bodyInHeader=not self.ownsMembers,
+                                   inline=not self.ownsMembers))
+
         methods.append(ClassMethod("ToJSVal", "bool", [
                                    Argument("JSContext*", "cx"),
                                    Argument("JS::Handle<JSObject*>", "scopeObj"),
                                    Argument("JS::MutableHandle<JS::Value>", "rval")
         ], body=CGSwitch("mType", toJSValCases,
                          default=CGGeneric("return false;")).define(), const=True))
 
         constructors = [ctor]
@@ -7276,18 +7281,18 @@ class CGUnionStruct(CGThing):
         return CGClass(selfName,
                        bases=bases,
                        members=members,
                        constructors=constructors,
                        methods=methods,
                        disallowCopyConstruction=disallowCopyConstruction,
                        extradeclarations=friend,
                        destructor=ClassDestructor(visibility="public",
-                                                  body=dtor,
-                                                  bodyInHeader=not self.ownsMembers),
+                                                  body="Uninit();",
+                                                  bodyInHeader=True),
                        enums=[ClassEnum("Type", enumValues, visibility="private")],
                        unions=[ClassUnion("Value", unionValues, visibility="private")])
 
     def getConversionToJS(self, templateVars, type):
         assert not type.nullable() # flatMemberTypes never has nullable types
         val = "mValue.m%(name)s.Value()" % templateVars
         wrapCode = wrapForType(
             type, self.descriptorProvider,