diff --git a/lib/formats/json.c b/lib/formats/json.c
index be5faae5a..0328c6ec4 100644
--- a/lib/formats/json.c
+++ b/lib/formats/json.c
@@ -261,6 +261,9 @@ int json_print(struct io *io, struct sample *smps[], unsigned cnt)
 		fputc('\n', f);
 
 		json_decref(json);
+
+		if (ret)
+			return ret;
 	}
 
 	return i;
diff --git a/lib/formats/json_reserve.c b/lib/formats/json_reserve.c
index 632c79414..c4e4f2d75 100644
--- a/lib/formats/json_reserve.c
+++ b/lib/formats/json_reserve.c
@@ -45,8 +45,8 @@ int json_reserve_pack_sample(struct io *io, json_t **j, struct sample *smp)
 	json_data = json_array();
 
 	for (int i = 0; i < smp->length; i++) {
-		if (smp->destination)
-			sig = (struct signal *) list_at(&smp->destination->signals, i);
+		if (io->output.signals)
+			sig = (struct signal *) list_at_safe(io->output.signals, i);
 		else
 			sig = NULL;
 
@@ -134,17 +134,21 @@ int json_reserve_unpack_sample(struct io *io, json_t *json_smp, struct sample *s
 		if (ret)
 			return -1;
 
-		idx = signal_get_offset(name, smp->source);
-		if (idx < 0)
-			return -1;
+		void *s = list_lookup(io->input.signals, name);
+		if (s)
+			idx = list_index(io->input.signals, s);
+		else {
+			ret = sscanf(name, "signal_%d", &idx);
+			if (ret != 1)
+				continue;
+		}
 
-		if (idx >= smp->capacity)
-			continue;
+		if (idx < smp->capacity) {
+			smp->data[idx].f = value;
 
-		if (idx >= smp->length)
-			smp->length = idx;
-
-		smp->data[idx].f = value;
+			if (idx >= smp->length)
+				smp->length = idx;
+		}
 	}
 
 	if (smp->length > 0)
@@ -206,7 +210,7 @@ int json_reserve_sscan(struct io *io, char *buf, size_t len, size_t *rbytes, str
 	if (rbytes)
 		*rbytes = err.position;
 
-	return ret;
+	return 1;
 }
 
 int json_reserve_print(struct io *io, struct sample *smps[], unsigned cnt)
@@ -225,6 +229,9 @@ int json_reserve_print(struct io *io, struct sample *smps[], unsigned cnt)
 		fputc('\n', f);
 
 		json_decref(json);
+
+		if (ret)
+			return ret;
 	}
 
 	return i;
diff --git a/lib/io.c b/lib/io.c
index 8d80c1351..012a3060a 100644
--- a/lib/io.c
+++ b/lib/io.c
@@ -81,6 +81,10 @@ skip:		bytes = getdelim(&io->input.buffer, &io->input.buflen, io->delimiter, f);
 
 int io_init(struct io *io, struct format_type *fmt, struct node *n, int flags)
 {
+	int ret;
+
+	assert(io->state == STATE_DESTROYED);
+
 	io->_vt = fmt;
 	io->_vd = alloc(fmt->size);
 
@@ -101,13 +105,21 @@ int io_init(struct io *io, struct format_type *fmt, struct node *n, int flags)
 		io->output.signals = &n->signals;
 	}
 
-	return io->_vt->init ? io->_vt->init(io) : 0;
+	ret = io->_vt->init ? io->_vt->init(io) : 0;
+	if (ret)
+		return ret;
+
+	io->state = STATE_INITIALIZED;
+
+	return 0;
 }
 
 int io_destroy(struct io *io)
 {
 	int ret;
 
+	assert(io->state == STATE_CLOSED || io->state == STATE_INITIALIZED);
+
 	ret = io->_vt->destroy ? io->_vt->destroy(io) : 0;
 	if (ret)
 		return ret;
@@ -116,6 +128,8 @@ int io_destroy(struct io *io)
 	free(io->input.buffer);
 	free(io->output.buffer);
 
+	io->state = STATE_DESTROYED;
+
 	return 0;
 }
 
@@ -295,10 +309,10 @@ int io_open(struct io *io, const char *uri)
 	if (ret)
 		return ret;
 
-	io_header(io);
-
 	io->state = STATE_OPENED;
 
+	io_header(io);
+
 	return 0;
 }
 
@@ -447,12 +461,12 @@ int io_sscan(struct io *io, char *buf, size_t len, size_t *rbytes, struct sample
 {
 	struct format_type *fmt = io->_vt;
 
-	return fmt->sscan ? fmt->sscan(buf, len, rbytes, smps, cnt, io->flags) : -1;
+	return fmt->sscan ? fmt->sscan(io, buf, len, rbytes, smps, cnt) : -1;
 }
 
 int io_sprint(struct io *io, char *buf, size_t len, size_t *wbytes, struct sample *smps[], unsigned cnt)
 {
 	struct format_type *fmt = io->_vt;
 
-	return fmt->sprint ? fmt->sprint(buf, len, wbytes, smps, cnt, io->flags) : -1;
+	return fmt->sprint ? fmt->sprint(io, buf, len, wbytes, smps, cnt) : -1;
 }