Why treat InternalAuthenticationServiceException as 401 error?

When exceptions occur in BasicAuthenticationFilter, most of them should be treated as 401 error.
But InternalAuthenticationServiceException is an internal error, why not treat it is as 500 error?
I think the commence function in BasicAuthenticationEntryPoint should be like as follow:

public void commence(HttpServletRequest request, HttpServletResponse response,
            AuthenticationException authException) throws IOException, ServletException {
        response.addHeader("WWW-Authenticate", "Basic realm=\"" + realmName + "\"");
        if (authException instanceof InternalAuthenticationServiceException) {
            logger.error("Internal server error. ", authException);
            response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, authException.getMessage());
        }
        else {
            response.sendError(HttpServletResponse.SC_UNAUTHORIZED, authException.getMessage());
        }
    }

Start by reading BasicAuthenticationFilter and BasicAuthenticationEntryPoint, focusing on how authentication exceptions reach commence. Check existing tests for Basic authentication entry-point responses, then verify that InternalAuthenticationServiceException and other authentication failures produce the intended status codes.