diff --git a/include/villas/list.h b/include/villas/list.h
index 7d15bf941..07fd58e9f 100644
--- a/include/villas/list.h
+++ b/include/villas/list.h
@@ -16,6 +16,8 @@
 #include <stdbool.h>
 #include <pthread.h>
 
+#include "common.h"
+
 #define LIST_CHUNKSIZE		16
 
 /** Static list initialization */
@@ -23,7 +25,8 @@
 	.array = NULL,				\
 	.length = 0,				\
 	.capacity = 0,				\
-	.lock = PTHREAD_MUTEX_INITIALIZER	\
+	.lock = PTHREAD_MUTEX_INITIALIZER,	\
+	.state = STATE_INITIALIZED		\
 }
 				
 #define list_length(list)	((list)->length)
@@ -49,6 +52,7 @@ struct list {
 	size_t capacity;	/**< Size of list::array in elements */
 	size_t length;		/**< Number of elements of list::array which are in use */
 	pthread_mutex_t lock;	/**< A mutex to allow thread-safe accesses */
+	enum state state;	/**< The state of this list. */
 };
 
 /** Initialize a list.
diff --git a/lib/api.c b/lib/api.c
index 86b5ad668..4722dd7db 100644
--- a/lib/api.c
+++ b/lib/api.c
@@ -126,6 +126,9 @@ int api_http_protocol_cb(struct lws *wsi, enum lws_callback_reasons reason, void
 		case LWS_CALLBACK_ESTABLISHED: {
 			struct web *w = (struct web *) lws_context_user(lws_get_context(wsi));
 			
+			if (w->api == NULL)
+				return -1; /** @todo return error message */
+			
 			api_session_init(s, w->api, API_MODE_WS);
 			break;
 		}
@@ -245,8 +248,7 @@ int api_init(struct api *a, struct super_node *sn)
 
 int api_destroy(struct api *a)
 {
-	if (a->state == STATE_STARTED)
-		return -1;
+	assert(a->state != STATE_STARTED);
 
 	a->state = STATE_DESTROYED;
 
diff --git a/lib/list.c b/lib/list.c
index 1d938e882..0a95ed9bf 100644
--- a/lib/list.c
+++ b/lib/list.c
@@ -10,6 +10,7 @@
 #include <string.h>
 
 #include "list.h"
+#include "utils.h"
 
 /* Compare functions */
 static int cmp_lookup(const void *a, const void *b) {
@@ -32,18 +33,22 @@ static int cmp_sort(const void *a, const void *b, void *thunk) {
 
 void list_init(struct list *l)
 {
+	assert(l->state == STATE_DESTROYED);
+
 	pthread_mutex_init(&l->lock, NULL);
 
 	l->length = 0;
 	l->capacity = 0;
-
 	l->array = NULL;
+	l->state = STATE_INITIALIZED;
 }
 
 int list_destroy(struct list *l, dtor_cb_t destructor, bool release)
 {
 	pthread_mutex_lock(&l->lock);
 
+	assert(l->state != STATE_DESTROYED);
+
 	list_foreach(void *p, l) {
 		if (destructor)
 			destructor(p);
@@ -61,12 +66,16 @@ int list_destroy(struct list *l, dtor_cb_t destructor, bool release)
 	pthread_mutex_unlock(&l->lock);
 	pthread_mutex_destroy(&l->lock);
 	
+	l->state = STATE_DESTROYED;
+	
 	return 0;
 }
 
 void list_push(struct list *l, void *p)
 {
 	pthread_mutex_lock(&l->lock);
+
+	assert(l->state == STATE_INITIALIZED);
 	
 	/* Resize array if out of capacity */
 	if (l->length >= l->capacity) {
@@ -86,6 +95,8 @@ void list_remove(struct list *l, void *p)
 
 	pthread_mutex_lock(&l->lock);
 
+	assert(l->state == STATE_INITIALIZED);
+
 	for (int i = 0; i < l->length; i++) {
 		if (l->array[i] == p)
 			removed++;
@@ -114,6 +125,8 @@ int list_count(struct list *l, cmp_cb_t cmp, void *ctx)
 
 	pthread_mutex_lock(&l->lock);
 
+	assert(l->state == STATE_INITIALIZED);
+
 	list_foreach(void *e, l) {
 		if (cmp(e, ctx) == 0)
 			c++;
@@ -129,6 +142,8 @@ void * list_search(struct list *l, cmp_cb_t cmp, void *ctx)
 	void *e;
 
 	pthread_mutex_lock(&l->lock);
+
+	assert(l->state == STATE_INITIALIZED);
 	
 	list_foreach(e, l) {
 		if (!cmp(e, ctx))
@@ -146,6 +161,8 @@ void list_sort(struct list *l, cmp_cb_t cmp)
 {
 	pthread_mutex_lock(&l->lock);
 
+	assert(l->state == STATE_INITIALIZED);
+
 	qsort_r(l->array, l->length, sizeof(void *), cmp_sort, (void *) cmp);
 
 	pthread_mutex_unlock(&l->lock);
diff --git a/lib/log.c b/lib/log.c
index de554f686..846fefd1c 100644
--- a/lib/log.c
+++ b/lib/log.c
@@ -97,8 +97,7 @@ int log_start(struct log *l)
 
 int log_stop(struct log *l)
 {
-	if (l->state != STATE_STARTED)
-		return -1;
+	assert(l->state == STATE_STARTED);
 	
 	l->state = STATE_STOPPED;
 	
@@ -107,8 +106,7 @@ int log_stop(struct log *l)
 
 int log_destroy(struct log *l)
 {
-	if (l->state == STATE_STARTED)
-		return -1;
+	assert(l->state != STATE_STARTED);
 	
 	l->state = STATE_DESTROYED;
 	
@@ -232,7 +230,7 @@ void debug(long class, const char *fmt, ...)
 	int lvl = class &  0xFF;
 	int fac = class & ~0xFF;
 	
-	assert(log);
+	assert(log != NULL);
 
 	if (((fac == 0) || (fac & log->facilities)) && (lvl <= log->level)) {
 		va_start(ap, fmt);
@@ -245,7 +243,7 @@ void info(const char *fmt, ...)
 {
 	va_list ap;
 	
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	log_vprint(log, LOG_LVL_INFO, fmt, ap);
@@ -256,7 +254,7 @@ void warn(const char *fmt, ...)
 {
 	va_list ap;
 	
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	log_vprint(log, LOG_LVL_WARN, fmt, ap);
@@ -267,7 +265,7 @@ void stats(const char *fmt, ...)
 {
 	va_list ap;
 
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	log_vprint(log, LOG_LVL_STATS, fmt, ap);
@@ -278,7 +276,7 @@ void error(const char *fmt, ...)
 {
 	va_list ap;
 	
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	log_vprint(log, LOG_LVL_ERROR, fmt, ap);
@@ -292,7 +290,7 @@ void serror(const char *fmt, ...)
 	va_list ap;
 	char *buf = NULL;
 
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	vstrcatf(&buf, fmt, ap);
@@ -309,7 +307,7 @@ void cerror(config_setting_t *cfg, const char *fmt, ...)
 	va_list ap;
 	char *buf = NULL;
 
-	assert(log);
+	assert(log != NULL);
 
 	va_start(ap, fmt);
 	vstrcatf(&buf, fmt, ap);
diff --git a/lib/node.c b/lib/node.c
index 5e365ed9f..bd4fbe0be 100644
--- a/lib/node.c
+++ b/lib/node.c
@@ -15,8 +15,7 @@
 
 int node_init(struct node *n)
 {
-	if (n->state != STATE_DESTROYED)
-		return -1;
+	assert(n->state == STATE_DESTROYED);
 	
 	n->state = STATE_INITIALIZED;
 
@@ -54,8 +53,7 @@ int node_parse(struct node *n, config_setting_t *cfg)
 
 int node_check(struct node *n)
 {
-	if (n->state != STATE_INITIALIZED || n->state != STATE_PARSED)
-		return -1;
+	assert(n->state != STATE_DESTROYED);
 
 	if (n->vectorize <= 0)
 		error("Invalid `vectorize` value %d for node %s. Must be natural number!", n->vectorize, node_name(n));
@@ -73,8 +71,7 @@ int node_start(struct node *n)
 {
 	int ret;
 	
-	if (n->state != STATE_CHECKED)
-		return -1;
+	assert(n->state == STATE_CHECKED);
 
 	info("Starting node %s", node_name_long(n));
 	{ INDENT
@@ -93,8 +90,7 @@ int node_stop(struct node *n)
 {
 	int ret;
 
-	if (n->state != STATE_STARTED)
-		return -1;
+	assert(n->state == STATE_STARTED);
 
 	info("Stopping node %s", node_name(n));
 	{ INDENT
@@ -109,8 +105,7 @@ int node_stop(struct node *n)
 
 int node_destroy(struct node *n)
 {
-	if (n->state == STATE_STARTED)
-		return -1;
+	assert(n->state != STATE_DESTROYED && n->state != STATE_STARTED);
 
 	if (n->_vt->destroy)
 		n->_vt->destroy(n);
diff --git a/lib/node_type.c b/lib/node_type.c
index bbbfd4f1e..83fd068af 100644
--- a/lib/node_type.c
+++ b/lib/node_type.c
@@ -18,8 +18,7 @@ int node_type_start(struct node_type *vt, int argc, char *argv[], config_setting
 {
 	int ret;
 	
-	if (vt->state == STATE_STARTED)
-		return -1;
+	assert(vt->state != STATE_STARTED);
 
 	info("Initializing " YEL("%s") " node type", plugin_name(vt));
 	{ INDENT
@@ -36,8 +35,7 @@ int node_type_stop(struct node_type *vt)
 {
 	int ret;
 	
-	if (vt->state != STATE_STARTED)
-		return -1;
+	assert(vt->state == STATE_STARTED);
 
 	info("De-initializing " YEL("%s") " node type", plugin_name(vt));
 	{ INDENT
diff --git a/lib/plugin.c b/lib/plugin.c
index 4341cbbb1..c5c86e251 100644
--- a/lib/plugin.c
+++ b/lib/plugin.c
@@ -9,7 +9,7 @@
 #include "plugin.h"
 
 /** Global list of all known plugins */
-struct list plugins;
+struct list plugins = LIST_INIT();
 
 int plugin_init(struct plugin *p, char *name, char *path)
 {
@@ -36,8 +36,7 @@ int plugin_unload(struct plugin *p)
 {
 	int ret;
 	
-	if (p->state != STATE_LOADED)
-		return -1;
+	assert(p->state == STATE_LOADED);
 	
 	ret = dlclose(p->handle);
 	if (ret)
@@ -50,8 +49,7 @@ int plugin_unload(struct plugin *p)
 
 int plugin_destroy(struct plugin *p)
 {
-	if (p->state == STATE_LOADED)
-		plugin_unload(p);
+	assert(p->state != STATE_DESTROYED && p->state != STATE_LOADED);
 
 	if (p->path)
 		free(p->path);
diff --git a/lib/pool.c b/lib/pool.c
index 5ec74a95c..e2398d875 100644
--- a/lib/pool.c
+++ b/lib/pool.c
@@ -14,8 +14,7 @@ int pool_init(struct pool *p, size_t cnt, size_t blocksz, const struct memtype *
 {
 	int ret;
 	
-	if (p->state != STATE_DESTROYED)
-		return -1;
+	assert(p->state == STATE_DESTROYED);
 
 	/* Make sure that we use a block size that is aligned to the size of a cache line */
 	p->alignment = kernel_get_cacheline_size();
@@ -45,8 +44,7 @@ int pool_destroy(struct pool *p)
 {
 	int ret;
 
-	if (p->state != STATE_INITIALIZED)
-		return -1;
+	assert(p->state == STATE_INITIALIZED);
 
 	queue_destroy(&p->queue);	
 
diff --git a/lib/queue.c b/lib/queue.c
index 408c63d83..10ef9ec3e 100644
--- a/lib/queue.c
+++ b/lib/queue.c
@@ -38,8 +38,7 @@
 /** Initialize MPMC queue */
 int queue_init(struct queue *q, size_t size, const struct memtype *mem)
 {
-	if (q->state != STATE_DESTROYED)
-		return -1;
+	assert(q->state == STATE_DESTROYED);
 
 	/* Queue size must be 2 exponent */
 	if (!IS_POW2(size)) {
@@ -69,9 +68,8 @@ int queue_destroy(struct queue *q)
 {
 	int ret = 0;
 
-	if (q->state != STATE_INITIALIZED)
-		return -1;
-	
+	assert(q->state == STATE_INITIALIZED);
+
 	ret = memory_free(q->mem, q->buffer, (q->buffer_mask + 1) * sizeof(q->buffer[0]));
 
 	if (ret == 0)
diff --git a/lib/web.c b/lib/web.c
index f3f7bb4df..601f2c288 100644
--- a/lib/web.c
+++ b/lib/web.c
@@ -191,15 +191,18 @@ int web_start(struct web *w)
 
 int web_stop(struct web *w)
 {
+	assert(w->state == STATE_STARTED);
+
 	lws_cancel_service(w->context);
+	
+	w->state = STATE_STOPPED;
 
 	return 0;
 }
 
 int web_destroy(struct web *w)
 {
-	if (w->state == STATE_STARTED)
-		return -1;
+	assert(w->state != STATE_DESTROYED && w->state != STATE_STARTED);
 	
 	lws_context_destroy(w->context);
 
@@ -210,5 +213,7 @@ int web_destroy(struct web *w)
 
 int web_service(struct web *w)
 {
+	assert(w->state == STATE_STARTED);
+
 	return lws_service(w->context, 10);
 }