diff --git a/sys.c b/sys.c
index 19bbeb38e8f26ca577359bee68deb661a10f8b99..b3cf6b15bd7e7c01ff29ebe1c84e90929a7bbb4a 100644
--- a/sys.c
+++ b/sys.c
@@ -82,6 +82,7 @@ static oop_source_sys *verify_source(oop_source *source) {
 static void sys_on_fd(oop_source *source,int fd,oop_event ev,
                       oop_call_fd *f,void *v) {
 	oop_source_sys *sys = verify_source(source);
+	assert(NULL != f && "callback must be non-NULL");
 	if (fd >= sys->num_files) {
 		int i,j,num_files = 1 + fd;
 		sys_file *files = oop_malloc(num_files * sizeof(sys_file));
@@ -117,6 +118,7 @@ static void sys_on_time(oop_source *source,struct timeval tv,
 	oop_source_sys *sys = verify_source(source);
 	struct sys_time **p = &sys->time_queue;
 	struct sys_time *time = oop_malloc(sizeof(struct sys_time));
+	assert(NULL != f && "callback must be non-NULL");
 	if (NULL == time) return; /* ugh */
 	time->tv = tv;
 	time->f = f;
@@ -185,6 +187,7 @@ static void sys_on_signal(oop_source *source,int sig,
                           oop_call_signal *f,void *v) {
 	oop_source_sys *sys = verify_source(source);
 	struct sys_signal_handler *handler = oop_malloc(sizeof(*handler));
+	assert(NULL != f && "callback must be non-NULL");
 	if (NULL == handler) return; /* ugh */
 
 	assert(sig > 0 && sig < OOP_NUM_SIGNALS && "invalid signal number");
diff --git a/www.c b/www.c
index 3dabdef0163d8e08fadd839bf1e6665922a427ad..40ff6fab31bf5892a24e73e78abc7aa0e8670c1b 100644
--- a/www.c
+++ b/www.c
@@ -115,7 +115,7 @@ static int reg(SOCKET sock,HTEventType type,HTEvent *www_event) {
 		oop_type = OOP_WRITE;
 		break;
 	case HTEvent_INDEX(HTEvent_OOB):
-		// XXX: we don't handle this; does anything use it?
+		/* XXX: we don't handle this; does anything use it? */
 		return HT_ERROR;
 	default:
 		assert(0 && "invalid HTEvent type specified");
@@ -158,7 +158,7 @@ static int unreg(SOCKET sock,HTEventType type) {
 		oop_type = OOP_WRITE;
 		break;
 	case HTEvent_INDEX(HTEvent_OOB):
-		// XXX: we don't handle this; does anything use it?
+		/* XXX: we don't handle this; does anything use it? */
 		return HT_ERROR;
 	default:
 		assert(0 && "invalid HTEvent type specified");