--- a/xpcom/idl-parser/typelib.py
+++ b/xpcom/idl-parser/typelib.py
@@ -89,17 +89,19 @@ def build_interface(iface, ifaces):
             else:
                   tag = TypeMap[type.name]
                   isPtr = (tag == xpt.Type.Tags.char_ptr or tag == xpt.Type.Tags.wchar_t_ptr)
                   return xpt.SimpleType(tag,
                                         pointer=isPtr,
                                         reference=False)
 
         if isinstance(type, xpidl.Array):
-            return xpt.ArrayType(get_type(type.type, calltype), size_is,
+            # NB: For an Array<T> we pass down the iid_is to get the type of T.
+            #     This allows Arrays of InterfaceIs types to work.
+            return xpt.ArrayType(get_type(type.type, calltype, iid_is), size_is,
                                  #XXXkhuey length_is duplicates size_is (bug 677788),
                                  size_is)
 
         if isinstance(type, xpidl.Interface) or isinstance(type, xpidl.Forward):
             xptiface = None
             for i in ifaces:
                 if i.name == type.name:
                     xptiface = i