enum MyTestEnum { "a", "b" }; callback interface Foo { void passOptionalEnum(optional MyTestEnum arg); }; Seems vaguely similar to bug 841429. The problem seems to be declaring a variable |arg.Value()_str|, which is just bogus. JSString* arg.Value()_str = JS_NewStringCopyN(cx, MyTestEnumValues::strings[uint32_t(arg.Value())].value, MyTestEnumValues::strings[uint32_t(arg.Value())].length); ^ ; /Users/amccreight/mz/cent/obj-dbg/dom/bindings/JSImplBinding.cpp:48:15: error: member reference type 'JSString *' is a pointer; maybe you meant to use '->'? if (!arg.Value()_str) { ~~~^ -> /Users/amccreight/mz/cent/obj-dbg/dom/bindings/JSImplBinding.cpp:48:15: error: incomplete definition of type 'JSString' if (!arg.Value()_str) { ~~~^ ../../dist/include/jspubtd.h:191:45: note: forward declaration of 'JSString' class JSString; ^ /Users/amccreight/mz/cent/obj-dbg/dom/bindings/JSImplBinding.cpp:52:36: error: member reference type 'JSString *' is a pointer; maybe you meant to use '->'? argv[0] = JS::StringValue(arg.Value()_str); ~~~^ -> /Users/amccreight/mz/cent/obj-dbg/dom/bindings/JSImplBinding.cpp:52:36: error: incomplete definition of type 'JSString' argv[0] = JS::StringValue(arg.Value()_str); ~~~^ ../../dist/include/jspubtd.h:191:45: note: forward declaration of 'JSString' class JSString; ^ 5 errors generated.