diff --git a/sys/src/libc/9sys/idn.c b/sys/src/libc/9sys/idn.c
index ad05890a8..3f807c4a2 100644
--- a/sys/src/libc/9sys/idn.c
+++ b/sys/src/libc/9sys/idn.c
@@ -191,6 +191,9 @@ idn2utf(char *name, char *buf, int nbuf)
 	Rune rb[Domlen], r;
 	int nc, nr, n;
 
+	if(nbuf < 1)
+		return -1;
+
 	cp = name;
 	dp = buf;
 	de = dp+nbuf-1;
@@ -231,6 +234,9 @@ utf2idn(char *name, char *buf, int nbuf)
 	Rune rb[Domlen], r;
 	int nc, nr, n;
 
+	if(nbuf < 1)
+		return -1;
+
 	dp = buf;
 	de = dp+nbuf-1;
 	cp = name;