In glib/gio/gdbus-2.0/codegen/codegen.py:

class CodeGenerator:
    def __init__(self, ifaces, namespace, interface_prefix, generate_objmanager, docbook_gen, h, c):
        ...
        if len(namespace) > 1:
            self.ns_upper = utils.camel_case_to_uscore(namespace).upper() + '_'
            self.ns_lower = utils.camel_case_to_uscore(namespace).lower() + '_'
        else:
            self.ns_upper = ''
            self.ns_lower = ''
        ...

The "if" condition should be: len(namespace) > 0