Someone

dbus-test.vala:5.2-5.45: warning: method `org.freedesktop.NetworkManager.foo' never used
        public abstract HashMap<string, Variant> foo() throws IOError;
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

** (valac:12566): CRITICAL **: vala_gvariant_module_get_basic_type_info: assertion `signature != NULL' failed
dbus-test.vala:5.18-5.41: error: GVariant deserialization of type `Gee.HashMap<string,GLib.Variant>' is not supported
        public abstract HashMap<string, Variant> foo() throws IOError;
                        ^^^^^^^^^^^^^^^^^^^^^^^^

** (valac:12566): CRITICAL **: vala_gvariant_module_get_basic_type_info: assertion `signature != NULL' failed
dbus-test.vala:5.18-5.41: error: GVariant serialization of type `Gee.HashMap<string,GLib.Variant>' is not supported
        public abstract HashMap<string, Variant> foo() throws IOError;
                        ^^^^^^^^^^^^^^^^^^^^^^^^
Compilation failed: 2 error(s), 1 warning(s)