diff --git a/src/channel.c b/src/channel.c
index 2f98396..36b4e44 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -87,7 +87,7 @@
 	if (len == 0)
 		return -1;
 
-	if (len > chn->buf->size) {
+	if (len < 0 || len > chn->buf->size) {
 		/* we can't write this chunk and will never be able to, because
 		 * it is larger than the buffer. This must be reported as an
 		 * error. Then we return -2 so that writers that don't care can
@@ -157,6 +157,9 @@
 	if (unlikely(channel_input_closed(chn)))
 		return -2;
 
+	if (len < 0)
+		return -3;
+
 	max = buffer_max_len(chn);
 	if (unlikely(len > max - buffer_len(chn->buf))) {
 		/* we can't write this chunk right now because the buffer is
diff --git a/src/chunk.c b/src/chunk.c
index 3c1cfdf..fbba6d5 100644
--- a/src/chunk.c
+++ b/src/chunk.c
@@ -100,7 +100,7 @@
 	va_list argp;
 	int ret;
 
-	if (!chk->str || !chk->size)
+	if (chk->len < 0 || !chk->str || !chk->size)
 		return 0;
 
 	va_start(argp, fmt);
@@ -125,6 +125,9 @@
 	int olen, free;
 	char c;
 
+	if (dst->len < 0)
+		return dst->len;
+
 	olen = dst->len;
 
 	for (i = 0; i < src->len; i++) {
@@ -166,6 +169,9 @@
 	int olen, free;
 	char c;
 
+	if (dst->len < 0)
+		return dst->len;
+
 	olen = dst->len;
 
 	for (i = 0; i < src->len; i++) {