diff --git a/daemon/call_flags.c b/daemon/call_flags.c
index dea662fbe..977300af2 100644
--- a/daemon/call_flags.c
+++ b/daemon/call_flags.c
@@ -18,7 +18,7 @@ static void call_ng_flags_str_list(const ng_parser_t *, parser_arg list,
 		void (*callback)(str *, unsigned int, helper_arg), helper_arg);
 static void call_ng_flags_list(const ng_parser_t *, parser_arg list,
 		void (*str_callback)(str *, unsigned int, helper_arg),
-		void (*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
+		const char *(*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
 		helper_arg);
 static void call_ng_flags_esc_str_list(str *s, unsigned int, helper_arg);
 
@@ -149,7 +149,7 @@ static void call_ng_flags_item_pair_ht_iter(str *key, unsigned int idx, helper_a
 		from_to[1] = *key;
 }
 
-static void call_ng_flags_item_pair_ht(const ng_parser_t *parser, parser_arg it, helper_arg arg) {
+static const char *call_ng_flags_item_pair_ht(const ng_parser_t *parser, parser_arg it, helper_arg arg) {
 	str from_to[2] = {0};
 
 	if (!parser->is_list(it))
@@ -166,10 +166,11 @@ static void call_ng_flags_item_pair_ht(const ng_parser_t *parser, parser_arg it,
 		*ht = str_case_value_ht_new();
 	t_hash_table_replace(*ht, s_copy_from, s_copy_to);
 
-	return;
+	return NULL;
 
 err:
 	ilog(LOG_WARN, "SDP manipulations: Ignoring invalid contents of string-pair list");
+	return NULL;
 }
 
 /**
@@ -338,7 +339,7 @@ INLINE void ng_t38_option(str *s, unsigned int idx, helper_arg arg) {
 
 static void call_ng_flags_list(const ng_parser_t *parser, parser_arg list,
 		void (*str_callback)(str *, unsigned int, helper_arg),
-		void (*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
+		const char *(*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
 		helper_arg arg)
 {
 	str s;
@@ -1154,8 +1155,9 @@ static void call_ng_parse_block_mode(str *s, enum block_dtmf_mode *output) {
 #endif
 
 static void call_ng_flags_freqs(const ng_parser_t *parser, parser_arg value, sdp_ng_flags *out);
-static void call_ng_flags_freqs_iter(const ng_parser_t *parser, parser_arg item, helper_arg arg) {
+static const char *call_ng_flags_freqs_iter(const ng_parser_t *parser, parser_arg item, helper_arg arg) {
 	call_ng_flags_freqs(parser, item, arg.flags);
+	return NULL;
 }
 static void call_ng_flags_freqs(const ng_parser_t *parser, parser_arg value, sdp_ng_flags *out) {
 	unsigned int val;
@@ -1243,7 +1245,7 @@ static void call_ng_codec(const ng_parser_t *parser, str *key, parser_arg value,
 	}
 }
 
-static void call_ng_codec_iter(const ng_parser_t *parser, parser_arg item, struct ng_media *media) {
+static const char *call_ng_codec_iter(const ng_parser_t *parser, parser_arg item, struct ng_media *media) {
 	// we support two types here:
 	// the "transform" method supplies an extended list of codecs, as a list of dicts
 	// the "create" method uses a list of strings, similar to codec->offer
@@ -1251,7 +1253,7 @@ static void call_ng_codec_iter(const ng_parser_t *parser, parser_arg item, struc
 		str s;
 		parser->get_str(item, &s);
 		call_ng_flags_esc_str_list(&s, 0, &media->codec_list);
-		return;
+		return NULL;
 	}
 
 	__auto_type codec = g_new0(struct ng_codec, 1);
@@ -1264,6 +1266,8 @@ static void call_ng_codec_iter(const ng_parser_t *parser, parser_arg item, struc
 
 	if (codec->input.payload_type == -1 || codec->output.payload_type == -1)
 		ilog(LOG_WARN, "Incomplete codec definition");
+
+	return NULL;
 }
 
 static void call_ng_endpoint(const ng_parser_t *parser, str *key, parser_arg value, struct ng_media *media) {
@@ -1315,10 +1319,11 @@ static void call_ng_media(const ng_parser_t *parser, str *key, parser_arg value,
 	}
 }
 
-static void call_ng_media_iter(const ng_parser_t *parser, parser_arg item, sdp_ng_flags *out) {
+static const char *call_ng_media_iter(const ng_parser_t *parser, parser_arg item, sdp_ng_flags *out) {
 	__auto_type media = g_new0(struct ng_media, 1);
 	t_queue_push_tail(&out->medias, media);
 	parser->dict_iter(parser, item, call_ng_media, media);
+	return NULL;
 }
 
 void call_ng_main_flags(const ng_parser_t *parser, str *key, parser_arg value, helper_arg arg) {
diff --git a/daemon/control_ng.c b/daemon/control_ng.c
index 917cbd362..7ca9cbc1a 100644
--- a/daemon/control_ng.c
+++ b/daemon/control_ng.c
@@ -123,24 +123,28 @@ static bool bencode_is_list(bencode_item_t *arg) {
 static bool bencode_is_int(bencode_item_t *arg) {
 	return arg->type == BENCODE_INTEGER;
 }
-static void bencode_list_iter(const ng_parser_t *parser, bencode_item_t *list,
+static const char *bencode_list_iter(const ng_parser_t *parser, bencode_item_t *list,
 		void (*str_callback)(str *key, unsigned int, helper_arg),
-		void (*item_callback)(const ng_parser_t *, bencode_item_t *, helper_arg),
+		const char *(*item_callback)(const ng_parser_t *, bencode_item_t *, helper_arg),
 		helper_arg arg)
 {
 	if (list->type != BENCODE_LIST)
-		return;
+		return NULL; // return error?
 	str s;
 	unsigned int idx = 0;
 	for (bencode_item_t *it = list->child; it; it = it->sibling) {
 		if (bencode_get_str(it, &s) && str_callback)
 			str_callback(&s, idx, arg);
-		else if (item_callback)
-			item_callback(parser, it, arg);
+		else if (item_callback) {
+			const char *err = item_callback(parser, it, arg);
+			if (err)
+				return err;
+		}
 		else
 			ilog(LOG_DEBUG, "Ignoring non-string value in list");
 		idx++;
 	}
+	return NULL;
 }
 static long long bencode_get_int(bencode_item_t *arg) {
 	return arg->value;
@@ -331,17 +335,17 @@ static bool json_dict_iter(const ng_parser_t *parser, JsonNode *input,
 
 	return true;
 }
-static void json_list_iter(const ng_parser_t *parser, JsonNode *list,
+static const char *json_list_iter(const ng_parser_t *parser, JsonNode *list,
 		void (*str_callback)(str *key, unsigned int, helper_arg),
-		void (*item_callback)(const ng_parser_t *parser, JsonNode *, helper_arg),
+		const char *(*item_callback)(const ng_parser_t *parser, JsonNode *, helper_arg),
 		helper_arg arg)
 {
 	if (json_node_get_node_type(list) != JSON_NODE_ARRAY)
-		return;
+		return NULL; // return error?
 
 	JsonArray *a = json_node_get_array(list);
 	if (!a)
-		return;
+		return NULL; // return error?
 
 	unsigned int l = json_array_get_length(a);
 	for (unsigned int i = 0; i < l; i++) {
@@ -353,9 +357,14 @@ static void json_list_iter(const ng_parser_t *parser, JsonNode *list,
 			if (s)
 				str_callback(STR_PTR(s), i, arg);
 		}
-		else
-			item_callback(parser, n, arg);
+		else {
+			const char *err = item_callback(parser, n, arg);
+			if (err)
+				return err;
+		}
 	}
+
+	return NULL;
 }
 static str *json_get_str(JsonNode *a, str *out) {
 	const char *s = json_node_get_string(a);
diff --git a/daemon/control_ng_flags_parser.c b/daemon/control_ng_flags_parser.c
index 3807d9e90..bae8b7c6f 100644
--- a/daemon/control_ng_flags_parser.c
+++ b/daemon/control_ng_flags_parser.c
@@ -147,13 +147,13 @@ static bool rtpp_dict_list_closing(rtpp_pos *pos) {
 
 	return true;
 }
-static void rtpp_list_iter(const ng_parser_t *parser, rtpp_pos *pos,
+static const char *rtpp_list_iter(const ng_parser_t *parser, rtpp_pos *pos,
 		void (*str_callback)(str *key, unsigned int, helper_arg),
-		void (*item_callback)(const ng_parser_t *, parser_arg, helper_arg), helper_arg arg)
+		const char *(*item_callback)(const ng_parser_t *, parser_arg, helper_arg), helper_arg arg)
 {
 	// list opener
 	if (!skip_char(&pos->cur, '['))
-		return;
+		return NULL; // return error here?
 
 	unsigned int idx = 0;
 
@@ -168,8 +168,11 @@ static void rtpp_list_iter(const ng_parser_t *parser, rtpp_pos *pos,
 
 		// does it start another list or dict?
 		if (pos->cur.s[0] == '[') {
-			if (item_callback)
-				item_callback(parser, pos, arg);
+			if (item_callback) {
+				const char *err = item_callback(parser, pos, arg);
+				if (err)
+					return err;
+			}
 			goto next;
 		}
 
@@ -182,8 +185,11 @@ static void rtpp_list_iter(const ng_parser_t *parser, rtpp_pos *pos,
 
 		if (str_callback)
 			str_callback(&pos->cur, idx++, arg);
-		else if (item_callback)
-			item_callback(parser, pos, arg);
+		else if (item_callback) {
+			const char *err = item_callback(parser, pos, arg);
+			if (err)
+				return err;
+		}
 		if (end)
 			break;
 		goto next;
@@ -193,7 +199,10 @@ next:
 		if (!str_token_sep(&pos->cur, &pos->remainder, ' '))
 			break;
 	}
+
+	return NULL;
 }
+
 static bool rtpp_dict_iter(const ng_parser_t *parser, rtpp_pos *pos,
 		void (*callback)(const ng_parser_t *, str *, parser_arg, helper_arg),
 		helper_arg arg)
diff --git a/daemon/redis.c b/daemon/redis.c
index b2b9d0f9b..418fd2cff 100644
--- a/daemon/redis.c
+++ b/daemon/redis.c
@@ -2034,7 +2034,7 @@ static int json_link_maps(call_t *c, struct redis_list *maps,
 	return 0;
 }
 
-static void json_build_ssrc_iter(const ng_parser_t *parser, parser_arg dict, helper_arg arg) {
+static const char *json_build_ssrc_iter(const ng_parser_t *parser, parser_arg dict, helper_arg arg) {
 	struct call_media *md = arg.md;
 
 	uint32_t ssrc = parser_get_ll(dict, "ssrc");
@@ -2053,6 +2053,8 @@ static void json_build_ssrc_iter(const ng_parser_t *parser, parser_arg dict, hel
 		payload_tracker_add(&se_out->tracker, parser_get_ll(dict, "out_payload_type"));
 		obj_put(&se_out->h);
 	}
+
+	return NULL;
 }
 
 static int json_build_ssrc(struct call_media *md, parser_arg arg) {
diff --git a/include/control_ng.h b/include/control_ng.h
index 3069b43a1..b92f1bb70 100644
--- a/include/control_ng.h
+++ b/include/control_ng.h
@@ -104,9 +104,9 @@ struct ng_parser {
 		void (*callback)(const ng_parser_t *, str *, parser_arg, helper_arg),
 		helper_arg);
 	bool (*is_list)(parser_arg);
-	void (*list_iter)(const ng_parser_t *, parser_arg input,
+	const char *(*list_iter)(const ng_parser_t *, parser_arg input,
 			void (*str_callback)(str *key, unsigned int, helper_arg),
-			void (*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
+			const char *(*item_callback)(const ng_parser_t *, parser_arg, helper_arg),
 			helper_arg);
 	str *(*get_str)(parser_arg, str *s);
 	int (*strcmp)(parser_arg, const char *);